Large subsurface sewage disposal systems

Chapter 22: Large Subsurface Sewage Disposal Systems

This chapter provides an overview of large subsurface sewage disposal systems with design flows greater than 10,000 L/d. It also provides a comparison of these systems to those smaller systems regulated by Part 8 of Division B of the Building Code (O. Reg. 350/06) made under the Building Code Act, 1992.

22.1 Applicable Legislation

Small (i.e., total daily design sanitary sewage flow of 10,000 L/d (2640 US gal/d) or less) individual or multiple subsurface sewage disposal systems, located wholly within the boundaries of the lot or parcel of land on which are located the residence(s), building(s) or facility/ies which they serve, are subject to the requirements of Part 8 of Division B of the Building Code (O. Reg. 350/06) made under the Building Code Act, 1992. This Act is administered by the Ontario Ministry of Municipal Affairs and Housing.

Under Part 8 of the Building Code, the means to determine the total daily design sewage flow are provided in Article 8.2.1.3. The values in Tables 8.2.1.3.A. and 8.2.1.3.B. represent sewage flow generation rates from residential occupancies and other specific facilities.

The design and construction of small subsurface sewage disposal systems, under the jurisdiction of the Building Code Act, 1992, should strictly adhere to standards contained in Part 8 of the Building Code relating to:

• Classification of sewage systems and site evaluation;
• Sewage design flows and clearance requirements;
• Types and design of tanks used to collect, treat, hold sanitary sewage; and
• The sewage subsurface disposal design, construction, operation and maintenance requirements.

All sewage works with a design capacity in excess of 10,000 L/d, including subsurface disposal systems, are subject to the requirements of Section 53 of the Ontario Water Resources Act (OWRA) administered by the Ontario Ministry of the Environment^{footnote 1[1]}. Subsurface disposal systems with a design capacity in excess of 10,000L/d are referred to as large subsurface sewage disposal systems (LSSDS).

The design of a LSSDS under OWRA jurisdiction is subject to the ministry engineering review and approval process (Section 1.5 – Ministry Approval Program for Sewage Works). The engineering review provides an evaluation of how the designer intends to meet the site-specific performance criteria established for the design life of the sewage works. The performance-based ministry engineering review is not prescriptive in nature and the required performance levels may be met using many alternatives, including new and innovative technologies.

The designer of the LSSDS is advised to consider, where appropriate and applicable, the design standards for small subsurface disposal systems contained in Part 8, Division B, of the Building Code. However, clearances or separation distances from large systems to such features as wells, surface water bodies and property boundaries need to be determined on a case-specific basis - see Section 22.5 - Assessment of Impact on Water Resources.

Ministry technical reviewers will examine the proponent’s assessment of effects on water resources. This review evaluates the hydrogeological aspects of the assessment such as the subsurface conditions at the site, the choice of “reasonable use” groundwater criteria at the downgradient property boundary, and the site-specific discharge criteria to ensure that the downgradient criteria are met. In cases where discharge of groundwater to surface water is a concern, a surface water assessment and review may also be required.

Depending on the location of the site, the Clean Water Act, associated regulations and source water planning may apply. If the proposed system is to be sited in a location within a source water protection “vulnerable area” as defined under the Clean Water Act, the designer is advised to consider and address the requirements of the Clean Water Act prior to proceeding with an OWRA application for approval. Consultation with the local Conservation Authority or Source Water Protection Authority is recommended to determine if this is a concern and if so what specific requirements need to be addressed.

22.2 General

In addition to the common use of subsurface sewage disposal (i.e., septic tank systems) for individual residences, there are many applications that use this method of sewage disposal for large buildings or a number of buildings. Large sewage systems of this type may include fairly extensive sewage collection systems and sewage treatment works and may be similar to systems from which the final effluent is discharged to surface water.

Some typical examples of large systems are those serving:

• Nursing homes, hotels, motels and institutions;
• Subdivisions;
• Mobile home parks and tent and trailer parks;
• Clubhouses;
• Churches;
• Recreational parks and centres;
• Industrial and commercial parks, establishments and plazas; and
• Residential condominiums where each sewage system serves several units.

Proposals of LSSDS are often considered due to non availability of a municipal sewer system. Where the municipal sewer services are in an adjacent area, but require either sewer extension or treatment plant expansion (or both) in order to service the new development, the designer should ensure that the proposal is not in conflict with zoning by laws or the Official Plans for the area.

While this guideline is not intended as a statement on land use, it is acknowledged that most applications of LSSDS are in areas not likely to be serviced by centralized municipal or communal sewage works in the future. When considering the use of large subsurface systems in these areas, it is especially important to take into account the individual and cumulative effects of existing and future uses of the adjacent land on the operation of the systems.

The sanitary servicing strategy for a large development should be based on a review of the following hierarchy of servicing alternatives:

• The potential of extending local municipal sewer systems or of pumping the sewage to a local municipal treatment system;
• The potential to expand existing communal sewage treatment facilities in the area and to service the development via these facilities;
• The potential of developing local communal sewage treatment facilities to service the proposed development;
• The use of an on-site sewage treatment system; and
• The availability of municipal sewage treatment facilities or other facilities that can legally accept septage from LSSDS.

22.3 Evaluation of Site Characteristics

The proposed site for a LSSDS needs to be evaluated more rigorously than a single-residence site because of the larger volume of sewage that is to be applied and the greater need to determine hydraulic gradients of the groundwater. The designer needs to ensure that a system that applies a larger hydraulic load to the subsurface over a greater area does not exceed the site’s capacity to accept it. Restrictive soil horizons that may inhibit deep percolation need to be identified before proceeding with design. Groundwater mounding analysis should be performed to determine whether the hydraulic loading to the saturated zone, rather than the loading to the infiltration surface, controls system sizing.

The site should be evaluated for conditions that might inhibit construction or proper operation of the LSSDS. With input from the groundwater specialist, the designer should predict the direction of groundwater flow and propose the most logical location for a drainfield/dispersal field. In most cases a large drainfield/dispersal system will need a large amount of owner controlled, downgradient area to fully treat and dilute the effluent. It is recommended that there be no on-site wells, existing or planned, downgradient of the dispersal field, and that the field and any on-site wells, existing or planned, be located such that the wells would not draw in water that is affected by sewage.

Siting of the LSSDS should be done as early as possible in the land development planning process such that the hydrogeological assessment can be used to determine the appropriate drainfield/dispersal site, able to treat and dilute the effluent, before the housing locations are established. A decision has to be made early on as to whether the housing development is going to be serviced by individual drinking water wells or a communal drinking water system. Locating houses with individual drinking water wells downgradient from LSSDS drainfields is not advised unless it can be shown that the situation is protective of drinking water quality. The designer should choose a potential dispersal system location that best accommodates the above considerations based on the predicted groundwater flow direction, existing drinking water supplies and the plan for future drinking water supply.

22.4 Soil Evaluation

The site evaluation should include digging test pits or drilling boreholes at the proposed drainfield location to reveal soil and groundwater conditions. Soil conditions should be properly evaluated before designing the drainfield.

The purpose of the soil evaluation is to:

• Determine the depth to the primary restricting layer (high groundwater level, impermeable soil or rock). At least 0.9 m (3.0 ft) separation distance is recommended. Issues regarding depth to rock may also arise relative to possible undesirable effects on water resources (Section 22.5 - Assessment of Impact on Water Resources);
• Determine the influence of any secondary restrictions (hard pans, abrupt textural changes, disturbed soil);
• Determine the infiltration rates;
• Determine the ability of soil to transmit the effluent from infiltration surface to deeper and distant layers (percolation/linear loading rate/groundwater mounding);
• Assess treatment abilities of the unsaturated soil (based on soil texture and structure). Preliminary assessment criteria are provided in Table 22-1; and
• Determine any construction related concerns (smearing, compaction).

Evaluation of soil should be performed by a soils professional. The first step in a soils evaluation involves reviewing the applicable soil survey information. The next step involves the examination of the soil in soil pits. Soil descriptions should be recorded. Enough test pits should be excavated and analyzed to adequately characterize the site and should be located within or near the system boundaries. It is important to locate test pits such that the disturbed soil will not interfere with the future absorption area. In addition samples from deep borings to a depth of 3 m (10 ft) or more below the water table should be evaluated to determine the hydraulic capabilities of these deeper materials. Soil description including soil texture, structure and colour from the test pits should be part of the design documentation.

22.5 Assessment of Impact On Water Resources

Developments in the science of contaminant hydrogeology and in environmental regulations and policies may require changes in the ministry’s approach to water resources impact assessments. Also, certain aspects of the prediction of effects on water resources are highly case-specific and site-specific. Therefore, pre-submission consultation with ministry groundwater and surface water staff at the local Regional Office is strongly advised.

The following guidance for water resource impact assessments uses concepts presented in ministry Guideline B-7, Incorporation of the Reasonable Use Concept into MOEE Groundwater Management Activities and Procedure B-7-1, Determination of Contaminant Limits and Attenuation Zones. These are commonly referred to as the “Reasonable Use Guideline”. While the guidance presented here is partly based on the Reasonable Use Guideline, an assessment performed in support of an application for approval of a LSSDS should refer primarily to the guidance given in the Section 22.5 - Assessment of Impact on Water Resources.

This guidance applies to those sewage systems which discharge to the subsurface and are governed by the requirements of Section 53 of the Ontario Water Resources Act, including sewage spray irrigation systems.

A water resources impact assessment is required to assess the risk of undesirable effects of the sewage, from the point where it enters the subsurface, on surrounding water bodies, water resources, and other users, including all groundwater and surface water that may be significantly affected. The focus of such an assessment is on the effect of the sewage constituents on the quality of waters relative to any function or use, potential or actual, of those waters. This assessment should take into account the design of a sewage works, especially as the design would affect effluent quality. In turn, the design of the works would need to minimize the risk of undesirable environmental effects.

Since the groundwater regime is the initial medium to receive sewage effluent, the primary technical review for these assessments will be done by a hydrogeologist in the ministry Technical Support Section at the local Regional Office. The degree of detail required for the assessment of surface water body effects is especially dependant on the attributes of receiving water bodies. Therefore, pre-submission consultation with ministry groundwater and surface water staff regarding potential surface water impacts is advised.

22.5.1 Ministry Documents

Other Ministry documents relevant to LSSDS approvals are:

• Guideline B-1 Water Management Policies, Guidelines, Provincial Water Quality Objectives of the Ministry of the Environment;
• Guideline B-7 Incorporation of the Reasonable Use Concept into MOEE Groundwater Management Activities;
• Procedure B-7-1 Determination of Contaminant Limits and Attenuation Zones;
• Wells Regulation Ontario Regulation 903, Revised Regulations of Ontario 1990;
• Clean Water Act; and
• Authorship of Water Resource Impact Assessment.

22.5.2 Authorship of Water Resource Impact Assessment

Technical submissions need to be prepared by a geoscientist, designated P.Eng. or P.Geo., qualified to prepare assessments of groundwater quality impacts and, where necessary, by persons qualified to prepare surface water quality impact assessments.

Technical submissions should contain the names, signatures, and qualifications/designations of the authors.

22.5.3 Pre-submission Consultation and Preliminary Groundwater Impact Assessment

For new developments, a proponent should undertake a preliminary assessment, utilizing existing data, to determine feasibility. Usually a proponent’s property can support some level of development relying upon commercially available on-site sewage treatment technology. The preliminary assessment should evaluate the subject property and the surrounding properties to determine what level of development can take place within the bounds of ministry requirements and an appropriate level of risk.

The preliminary assessment should be presented to Water Resources staff at the ministry’s Regional Office as part of pre-submission consultation. These preliminary steps are for the proponent’s benefit to avoid the expenditure of large sums of money in advancing a proposal that may be infeasible or uneconomical because of stringent effluent discharge criteria necessitated by the hydrogeological environment or the sensitivity of receiving surface waters.

Also in the case of a simple replacement or minor alteration of an infiltration facility, the proponent should contact the ministry to determine what degree of evaluation may be required. (Refer to Section 22.5.18 - Replacement of Infiltration Facilities.)

22.5.4 Scope of Detailed Water Resource Impact Assessment

In most cases a detailed groundwater impact assessment should be completed to adequately characterize the site and determine the anticipated impact of the development upon the environment and public health. Recommendations for the scope of a detailed groundwater impact assessment are outlined in Section 22.6 - Scope of Detailed Water Resources Impact Assessment in the Design of LSSDS.

22.5.5 Critical Contaminants

Most proposals for LSSDS involve domestic sewage. In the case of groundwater, the critical contaminant is typically nitrate. It should be assumed that all nitrite and ammonia will convert to nitrate. All water quality assessments should report the concentrations of these substances in units of “as Nitrogen” such as, “nitrate-N”, “nitrite-N” and “total ammonia-N”.

In the case of surface waters, the critical contaminants are usually ammonia and phosphorus. The ministry is proposing to establish an objective for nitrate in surface water. Should that be finalized, nitrate may become a critical contaminant for surface water too.

If the system is treating any other waste (e.g. industrial or commercial waste), or contains quantities of non-domestic wastes in excess of those typically found in domestic wastes, it is the responsibility of the proponent to evaluate the effluent quality in order to identify the critical contaminant(s).

22.5.6 Water Quality Limits

With respect to groundwater, the impact of a sewage system using subsurface discharge is assessed using a procedure derived from the ministry’s Guideline B-7, Incorporation of the Reasonable Use Concept into MOEE Groundwater Management Activities. It is generally assumed that the reasonable use of any groundwater is drinking water. Consequently, the allowable contaminant concentration limits in groundwater at a property boundary are normally based on relevant drinking water quality standards such as the Ontario Drinking Water Quality Standards (ODWQS).

Water quality limits for the purposes of water resources impact assessment are a fraction of the relevant drinking water standards. The maximum allowable concentration at the property boundary for a substance that originates in the sewage and that has an applicable drinking water quality limit, such as an ODWQS, is one quarter of a health-related limit and one half of an aesthetic limit. In a situation where the reasonable use of groundwater is drinking water, the maximum concentration for nitrate in groundwater affected by sewage effluent is 2.5 mg/L as N because the health-related ODWQS for nitrate is 10 mg/L as N. The equivalent concentrations for chloride, which is an aesthetic parameter, are 125 and 250 mg/L, respectively. Note that background concentrations are not used in the calculation of allowable concentrations for the purposes of the water resources impact assessment. However, determining background concentrations is normally necessary for monitoring purposes. (Refer to Section 22.5.7 - Existing and Background Concentrations and Section 22.5.8 - Prediction of Contaminant Attenuation for more information.)

Where the plume originating at the sewage system discharges to surface waters, whether these waters are on or off site, the assessment of impact would use procedures derived from Guideline B-1 Water Management Policies, Guidelines, Provincial Water Quality Objectives of the Ministry of the Environment. Although the Provincial Water Quality Objectives would be used in such assessments, the determinations of water quality limits are not as straightforward as for the drinking water case. The determination of limits is much more dependant on receiver-specific characteristics, and the ¼ and ½ fractions used in the drinking water case do not apply. Phosphorus and ammonia are likely to be contaminants of interest in the surface water case.

In certain cases, the applicant may wish to propose other reasonable uses or other water quality standards, such as where activities in the downgradient area are not and will not in the future be dependant on groundwater or on a particular use of groundwater. Discretion to accept or reject any such proposals lies with the ministry’s Regional Director. Very early pre-submission consultation is advised in such cases.

22.5.7 Existing and Background Concentrations

Existing and background concentrations of critical contaminants should generally be used only for reference, not for calculation of allowable water quality limits. Obtaining and analyzing samples and recording the concentrations is normally required as part of the impact assessment and may also be required as part of a long-term monitoring program for the purposes of differentiating between the effects of the subject sewage system and other sources. Groundwater sampling to determine existing background concentrations should be undertaken using monitors located on the site and with screened intervals within the hydrostratigraphic unit that will receive the effluent.

22.5.8 Prediction of Contaminant Attenuation

For the basic calculation of impact of effluent on groundwater quality at a receiver (i.e., a property boundary or a surface water body), a constant quantity of dilution, which is a surrogate for all attenuative mechanisms, should be used in most cases. That quantity is 250 mm (10 in) of water per year over the area of the contaminant plume, which is approximately the rate of infiltration of precipitation into a sand unit. It has been used as the amount of uncontaminated water that mixes with and dilutes a contaminant plume in a sand environment. Since soils with a finer texture are likely to be more attenuative than sand, but would likely allow less infiltration, the sand-related quantity is applied as a surrogate for all attenuative mechanisms to all soil textures.

In the case of a leaching bed, the area of the plume is the product of the width of the leaching bed transverse to the groundwater gradient and the distance from the upgradient edge of the leaching bed to the relevant receptor (that is, the leaching bed area is included in the plume area). For an exfiltration lagoon, the dimensions are similarly obtained from the footprint of the exfiltration area and the downgradient area. For a spray irrigation system, it is the dimensions of the area of spray application and the downgradient area.

The area of the plume depends on the width of the area of sewage infiltration, and the greater this width, the greater the dispersion of the sewage input to groundwater. Increasing the width of the sewage infiltration area and increasing the distance to the receptor of interest would both contribute to increasing attenuation.

The area/dilution approach described above may not always be appropriate, such as where effects on surface water are being considered, or where soils are of low permeability, or where the nature of the effluent differs significantly from typical household wastewater. Please refer to Section 22.5.11 - Attenuation of Phosphorus and Ammonia and to Section 22.5.14 - Low Permeability Environments. A basic groundwater dilution schematic and formulation is provided below.

Figure 22-1 - Sample schematic for a basic groundwater dilution calculation

With reference to Figure 22.1, estimates of the annual dilution volume (VA), the total volume of water (VT) and concentration at the property boundary (CPB) may be calculated as follows:

V_A = A_D × k

V_T = V_A + V_S

C_PB = (C_S × V_S) ⁄ V_T

Where:

V_A

annual dilution volume [m³ (US gal)];

A_D

dilution area [m² (ft²)];

V_T

total volume of water [m³ (US gal)];

V_S

annual sewage volume [m³ (US gal)];

C_PB

concentration at property boundary [mg/L (lb/US gal)];

C_S

concentration in sewage [mg/L (lb/US gal)]; and

k

0.25 m (SI) or 6.23 US gal/ft² (US).

The above calculation assumes a 250 mm (10 in) annual dilution precipitation rate (k).

22.5.9 Sewage Effluent Volumes

Sewage system design flows normally incorporate a safety margin and may therefore be greater than actual flows. A realistic estimate of the daily flows should be based on monitoring of similar, existing systems or, in the absence of such performance data, on accepted facility-based standards. In this case, a Certificate of Approval (C of A) granted by the ministry would stipulate a maximum flow and may require a program of monitoring of flows to ensure that the assessment assumptions are valid.

In the case of systems that are only occasionally used, such as seasonal-use facilities, it is assumed that the contaminant plume moves as a slug with advecting groundwater and that it affects water quality when it arrives at the receptor in the same way that a continuous source would. Therefore, an occasional-use system should be assessed as though it was a continuous-use system.

22.5.10 Effluent Quality

The proponent needs to determine the concentration of the contaminants of concern in the sewage effluent at the point of input to the infiltration surface. The proponent should acquire such information from documented monitoring of similar facilities. Where such information is not available, and sewage is expected to be a typical domestic type, an estimated nitrate-nitrogen concentration of 40 mg/L is generally acceptable.

Where a treatment facility is required to reduce the concentration of a contaminant in the sewage effluent to a specified level before discharge to the subsurface, documentation of the treatment system performance is required. In this case, monitoring of effluent quality would be needed. Contingency plans in case of failure may also be necessary, and would become conditions in a C of A.

22.5.11 Attenuation of Phosphorus and Ammonia

Phosphorus is of concern relative to surface water. Many geological materials have a high capacity to attenuate phosphorus by precipitation in the unsaturated zone and by adsorption below the water table. However, the attenuative capacity of a geological material is limited, and often reversible, which can result in significant phosphorus movement over the long term. In some settings, such as those with exposed fractured bedrock or with only a thin veneer of overburden, the phosphorus attenuation capacity may be quite low.

In the case of proposed sewage systems on the Precambrian Shield, the effects of phosphorus discharge to surface waters should be evaluated considering the requirements and recommendations in the document Lakeshore Capacity Assessment Handbook - Protecting Water Quality in Inland Lakes on Ontario’s Precambrian Shield - Consultation Draft, December 2007 or its replacement.

Un-ionized ammonia (NH₃) is also a concern. In cases where anaerobic conditions are maintained between the sewage system and the surface water body, ammonium may not be converted to nitrate before it discharges to surface water.

The potential for surface water impact increases as the distance to the point of plume discharge to the surface water decreases. In most cases, a separation distance of 300 metres (980 feet) between the area of sewage infiltration and the surface water body should be sufficient to ensure that there are no appreciable effects to surface water quality. However in certain situations such as where there are particularly sensitive receivers or where different surface water quality standards may apply, an assessment would be required.

Where the surface water body is relatively shallow, the assessment may need to include a three-dimensional evaluation of groundwater flow patterns to determine whether the plume actually discharges to the surface water body.

22.5.12 Microbial Pathogens

The risk to human health from pathogens originating in a sewage system is greater where the effluent has access to an aquifer with high groundwater flow velocities. This would occur where the aquifer is exposed at surface and consists of coarsely textured granular overburden or fractured or karstic bedrock, or where the soil overlying an aquifer is a fine-textured soil of limited thickness penetrated by root holes, desiccation cracks or other highly conductive features. In such groundwater settings, the potential movement of pathogens is to be taken into consideration in evaluating the adequacy of the setback from the downgradient property boundary. Monitoring for specific pathogens or indicator organisms may be required in settings where rapid off-site movement of pathogens in groundwater is possible.

The maximum acceptable microbial pathogen content in drinking water is nil. In order to ensure adequate attenuation, the subsurface travel time between the area of sewage infiltration and the boundary or receptor should be considered. Where higher velocities are likely, treatment to remove/inactivate human pathogens may be required.

22.5.13 Assessment of Impact in Shallow Bedrock Environments

Shallow bedrock environments often pose difficulties both with respect to the attenuation of sewage-derived contaminants and the characterization of groundwater flow. Some of the problems encountered in these environments are:

• Unpredictable flow patterns governed by fracture/bedding plane orientation and geometry;
• High groundwater flow velocities; and
• Low attenuation capabilities.

In shallow bedrock environments, the additional risk will need to be taken into account and special construction (e.g. additional material in constructed leaching beds) or additional assurances regarding effluent quality may be required to ensure compliance with relevant criteria. The higher groundwater flow velocity with increased pathogen survival rate may also require provision for treatment to remove/inactivate human pathogens.

22.5.14 Low Permeability Environments

Where it can be shown that the uppermost subsurface unit(s) at an infiltration facility have a vertical hydraulic conductivity of 10-5 cm/sec or less, is at least 10 metres (33 feet) thick and extends at least 100 m (330 ft) downgradient of the infiltration area, attenuation calculations may not be required. The assessment would however need to demonstrate the absence of higher permeability pathways in the lower permeability material.

In the case of a leaching bed, where there is such a thick, extensive and low permeability unit at surface, a raised bed may be needed. In the case of a sewage lagoon that will have a constructed or artificial liner designed to prevent leakage into the subsurface, the requirement for a water resources impact assessment may not apply. However, where a sewage lagoon is excavated into native soils, the risk of leakage may be sufficient to warrant a water resource assessment pursuant to this guideline. A proponent should engage the ministry Regional Office in pre-submission consultation at an early stage to address this issue.

22.5.15 Existing Subsurface Disposal Systems

Where there is an existing subsurface disposal system at a site, or at a nearby site in a sufficiently similar environment, and the plume from that system has had sufficient time to reach a stable condition on site, the assessment of the existing impact from that system would need to be used as part of the water resources impact assessment for the proposed system. Extrapolation from actual performance data is considered to be a more reliable approach than a prediction based on theoretical assumptions that are difficult to verify. The extent to which a water resources impact assessment can rely upon an assessment of existing systems will largely depend on the similarity between the existing and proposed systems and environments. This should be evaluated on a case-by-case basis.

22.5.16 Contaminant Attenuation Zones

Where there is insufficient distance between the infiltration facility and the downgradient property boundary to meet required water quality limits at a property boundary, the proponent may wish to negotiate a formal Contaminant Attenuation Zone (CAZ) or other instrument with the owner(s) of the downgradient property(s) to acquire sufficient area. A CAZ would need to be agreed among other parties such as the municipality and the ministry, and it would have to be written into documentation of land ownership and land use planning. The ministry will require that the owner of the property containing the CAZ register a Certificate of Requirement in the Land Registry. The Certificate of Requirement notifies anyone with an interest in the property of the presence of the CAZ.

22.5.17 Assessment of Impact on On-site Wells

In the case of groundwater, the primary focus of the water resources assessment is on off-site impacts. The potential for degradation of on-site well water is also a concern and needs to be assessed. Any on-site wells, whether in use or not, should be located and examined for contamination potential. On-site wells and other subsurface structures may act as a conduit for vertical contaminant movement. The location of infiltration area should be chosen to ensure that the quality of water from on-site wells is not compromised.

On-site wells need to be constructed, maintained and, if unused or subject to contamination, abandoned in accordance with O. Reg. 903.

The water resource impact assessment should:

• Identify all on-site water wells, whether in use or not, including information from Water Well Records, consultants’ reports and hearsay;
• Specify if the on-site water wells are or might be used as a source of drinking water;
• Review the location of the on-site water wells with respect to separation distances between wells and contaminant sources, as may be required by the Part 8 of Division B of the Building Code (O. Reg. 350/06) and Wells Regulation (O. Reg. 903);
• Assess the construction and integrity of the on-site water wells with respect to the requirements of O. Reg. 903;
• Determine the presence of any abandoned oil wells;
• Assess the potential for the well to be a conduit for contaminant movement; and
• Assess the potential for contamination of on-site water wells.

Where there is significant potential for contamination of an on-site well that is or may be used for human consumption, or where the well may act as a conduit for contaminant migration, proper abandonment of the well may be required as a condition of the C of A.

22.5.18 Replacement of Infiltration Facilities

In some cases where an existing subsurface sewage disposal system is being replaced and there is no increase in design capacity, a detailed water resources impact assessment may not be required. However, in some replacement scenarios, such as where there are incidents or complaints of contamination in any on-site or off-site wells or in groundwater or surface water, some form of assessment would likely be required. Pre-submission consultation with the Ministry’s Regional Office would be essential in such cases.

Proponents of replacement systems are encouraged to optimize the available on-site attenuation capacity of the site and apply reasonable technology in the replacement system design to reduce water resources impacts. Monitoring requirements for replacement sewage infiltration works should be evaluated on a case-by-case basis.

22.5.19 Post-Construction Monitoring and Contingency Programs

The water resource impact assessment should address the need for groundwater and surface water monitoring and contingency planning. These activities would be required in environments in which plume behaviour would be particularly difficult to characterize or where the consequences of failure of the predictions are of particular concern.

Circumstances in which a comprehensive monitoring program would likely be needed include the presence of:

• High groundwater flow velocities;
• Low attenuation capabilities;
• Specific effluent quality requirements; and
• Proximal receptors.

The components that may be required in a comprehensive monitoring program include:

• Clearly stated goals and objectives;
• A schedule for the monitoring of flows and quality of sewage at the point of input to the infiltrative surface;
• A plan for the location of monitoring wells in the plume, with provisions for future determination of whether the plume is delineated and whether additional wells are necessary;
• The designation of other wells and other water bodies for monitoring;
• A schedule for water level and water quality monitoring in wells and water bodies, including identification of the contaminants to be monitored;
• A schedule for continual assessment and reporting on compliance and efficacy of the program;
• A contingency plan for dealing with any problems that arise or that may reasonably be predicted to arise and a commitment to mitigate undesirable impacts;
• A protocol for routine and extraordinary reporting to the ministry; and
• A schedule of regular maintenance of the treatment works.

The monitoring reports prepared for the sewage works should present the results of the monitoring and maintenance in a manner that facilitates regulatory review. Monitoring reports should include, but not be limited to:

• Location and site maps (to scale) showing relevant features;
• Plume maps for important parameters, in three dimensions where necessary;
• Piezometric maps with interpreted flow directions and data points shown;
• Tabular summaries of data;
• Appended laboratory reports;
• Quality Assurance and Quality Control assessment of data and sample collection;
• Interpretation of attenuation processes within the plume and development of the plume relative to predictions made in the water resources impact assessment;
• Assessment of compliance with the relevant water quality criteria for groundwater and surface water and effluent quality criteria of sewage treatment processes, if any; and
• Recommendations for remedial action and changes to the monitoring program, including consideration of whether the plume is being adequately delineated and whether an expansion or other improvements in the monitoring program is needed.

The frequency of monitoring report submission should normally be less than annually, but will be decided on a case-by-case basis. More frequent submission may be required at the beginning of site monitoring or system operation. In cases where the actual submission of reports to the ministry is not required, the ministry may nevertheless require the owner of the system to produce and store routine reports and provide them to ministry staff on request or to send status letters confirming that required monitoring has been completed.

22.5.20 Geological Information in Support of Sewage Works Design

The Part 8 of Division B of the Building Code (O. Reg. 350/06) sets standards for small sewage systems and contains some design requirements that may also apply to LSSDS. The Code sets the range of the soil percolation time where a leaching bed could be constructed and includes design requirements for:

• Vertical separation from the high water table; and
• Vertical separation from bedrock or soils of low permeability.

The above requirements are to be considered and addressed as part of the overall system design with respect to LSSDS. The requirements in the Building Code represent the minimum standards acceptable for the installation of small systems (10,000 L/d or less). These standards may need to be increased for LSSDS and the designer of the LSSDS should assess the suitability of the use of these minimum standards for site-specific conditions.

22.6 Scope of Detailed Water Resources Impact Assessment in the Design of LSSDS

The following subsections provide bulleted lists outlining the scope of design information to be considered and included when conducting a detailed impact assessment for LSSDS designs.

22.6.1 Field Activities and Data Collection

A detailed water resources impact assessment may need to contain the following site-specific field work and data collection activities as well as any other information that prove to be necessary:

• Review of available Water Well Records, source water protection mapping etc., topographic maps and geological maps;
• Inspection of the site and its immediate vicinity with respect to land use, topography and vegetative cover as these might affect infiltration;
• Test pits, boreholes and associated logs;
• Examination of nearby road cuts, banks, erosional features, or excavations;
• Monitoring well installation;
• Hydraulic conductivity testing;
• Soil sampling and grain size analysis;
• Groundwater sampling and analysis for parameters including the critical contaminant (e.g. nitrate and nitrite) and other key contaminant indicator parameters (e.g. nitrate plus nitrite plus ammonia, electrical conductivity, dissolved oxygen, TOC, pH, phosphorus, TKN, sodium, chloride, metals, and potassium);
• Determination of static water elevations in wells;
• Inspection of the site and its immediate vicinity for evidence of permanent, intermittent or ephemeral water courses;
• Determination of relative elevations of surface water bodies on-site or in the immediate site vicinity; and
• Door-to-door survey in the site vicinity to determine water well use and characteristics.

22.6.2 Data Interpretation and Presentation

Interpretation and presentation of the data should include:

• Description of the methodology used to determine monitoring well elevations;
• Potentiometric plan maps with indications of groundwater flow directions;
• Presentation of hydrogeological information on cross-sections across the site and site vicinity (both parallel and transverse to the main groundwater flow direction);
• Summary tables of analytical data;
• Establishment of natural background and existing background levels of the critical contaminant(s);
• Determination of Reasonable Use of groundwater on the adjacent property; and
• Description of the expected lateral, vertical and longitudinal configuration of the plume.

22.6.3 Appended Information

Supporting information appended to the Groundwater Impact Assessment should include:

• Borehole/monitoring well logs;
• Door-to-door water well survey results;
• Laboratory analytical reports;
• Field analytical data;
• Confirmation from the analytical laboratory that it is certified for the analyses performed;
• Copies of water well records used in the assessment;
• Copies of permits for water takings in the site vicinity;
• Historical measurements of static well water elevations;
• Decommissioning procedures, proposed or already implemented, for boreholes, monitoring wells and test pits used in the investigation;
• Field notes for survey work done to establish monitoring well elevations or the report of a licensed surveyor; and
• Hydrogeology studies done in support of nearby development, which may be available in municipal planning offices.

22.6.4 Available Information

Information that should be retained by the consultant but available to the ministry upon request includes:

• Data collected during well development (as distinct from well purging) to demonstrate that the well is properly developed to enable representative groundwater samples to be obtained.

22.7 Design Considerations

22.7.1 LSSDS Versus Small Subsurface Sewage Disposal Systems

The principal distinction between a LSSDS subject to OWRA and small on-site sewage disposal systems subject to Part 8 of Division B of the Building Code (O. Reg. 350/06) standards lies in the area of complexity and the need for professional engineering design and construction supervision. A sewage system servicing a large development may have a much higher daily sewage flow, variations in peaking factors, and changing characteristics of the sewage. Similarly, it could service a large residential development and have similar sewage characteristics but require a sizeable collection system involving sanitary sewers, manholes and pumping stations. These types of systems need to be designed with consideration for site topography, hydraulic capacity, surface drainage, groundwater movement, and the overall impacts on the surrounding environment.

Every application for a large subsurface disposal system should be evaluated individually for specific site constraints. More extensive background studies may be required in support of a LSSDS to ensure compatibility with the site and protection of the environment. These may include, but are not limited to:

• Hydraulic dispersal to the soil of large volumes of sewage effluent may produce problems out of proportion to the increase of flows from a small to a large system. This is especially true where the soils underlying the drainfield present an increasing resistance to downward percolation as the depth increases, and where the lateral outflow potential of the more permeable upper soil layers limits site acceptability. The resistance to dispersal in the soil causes the sewage effluent to mound over the area of its application to a height that will create sufficient pressure to overcome the resistance. Dispersal in the underlying and surrounding soil without breakout to the surface is affected by:
  • The infiltration area covered by the drainfield. For the same sewage flow, a leaching bed constructed in a soil of low percolation time will require less area and will concentrate the applica¬tion of sewage effluent to the soil per unit of area, compared to a leaching bed treating the same sewage flow in a soil of higher percolation time;
  • The permeability and thickness of the underlying soil strata;
  • The depth to water table and its hydraulic gradient. The possibility of peak sewage flows occurring at the same time as high groundwater conditions, and the effect of heavy lawn sprinkling, or the diversion of surface waters, should be considered;
  • The direction of movement of the groundwater away from the bed, and whether or not the subsurface configuration at some point in that direction may restrict the outflow of sewage;
• The discharge in one location of a relatively large amount of contaminants into the soil makes their attenuation more difficult than in individual residential systems and emphasizes the need to assess the effects on the groundwater;
• The sewage collection system may be much more extensive than a single building sewer and include manholes, lift stations, pump chambers, and other structures;
• The assessment and computation of daily sewage flow requires knowledge of peak hourly and daily flows necessary for the proper selection and design of a treatment plant to meet these conditions;
• The site conditions such as site topography, drainage, high water table and the prevalence of rock or soils of low to unacceptable permeability may be restrictive to the location of large sewage systems because of the large area required. Where fissured rock is prevalent in the area, the location may be unacceptable because of the adverse effects on groundwater; and
• In large commercial systems, some constituents in the sewage, may be present in greater proportions than they are in residential sewage, and thus have a greater bearing on equipment selection and system design. An example is the concentration of washing detergents or disinfectants. Wastewater with a significant heavy metals content is not suited for treatment in a LSSDS.

The designer of a LSSDS should consider the need for:

• A requirement to register on title the need for a service contract or the need for including information regarding the servicing requirements;
• Any long term maintenance or operation agreements that may be required;
• A requirement for a spare area to allow for system enlargement should problems arise;
• Designation of any area which is set aside for expansion of the bed to handle expansion of the development;
• Any specific requirements respecting construction, such as supervision, certification by the professional engineer, or completion of as-constructed drawings, operation or maintenance;
• Any special conditions with respect to ongoing monitoring of the system;
• Any source water protection requirements, where applicable;
• Any limitations on the type of sewage flow to be treated; and
• Any restrictions on the use of the property or neighbouring properties with respect to the attenuation zone of the effluent plume.

Notwithstanding the above, the designer of the LSSDS is advised to consider, where appropriate and applicable, the design standards for small subsurface disposal systems contained in Part 8, Division B, of the Building Code. However, clearances or separation distances from large systems to such features as wells, surface water bodies and property boundaries need to be determined on a site-specific basis - see Section 22.5 - Assessment of Impact on Water Resources.

22.7.2 Sewage Characteristics

If the quality of sewage is similar to that of typical domestic wastewater, then it may be reasonable to design the system for the same type of sewage treatment as a typical small system under Part 8, Division B of the Building Code. If the LSSDS is proposed to service dry industry, commercial facilities, institutional development, restaurants, office buildings or a larger residential development, it will be necessary to assess both the sewage quality and flow characteristics.

There are some types of wastewater that may not be suitable to be treated with a LSSDS. These may include wastewater from automatic car washes, garage facilities, or some agricultural uses such as egg washing. LSSDS for these types of sewage may require complicated pretreatment or this type of wastewater may not be suitable for subsurface disposal.

Typical characteristics of undiluted (no infiltration) residential sewage are presented in Table 22-2.

22.7.3 Design Sewage Flows

The computation of the design sewage flow for a large sewage system will vary according to the nature of the development to be served. Recommendations concerning design flows are given in Section 5.5.2 – Design Sewage Flows. Calculation of design peak daily and design peak hourly flows will be necessary to determine the requirements of all parts of the sewage system. Collection systems for LSSDS may consist of single building drains from sites such as schools, or may consist of manholes, gravity sewers, and sewage pumping stations if from multi-structured facilities such as commercial malls. These collection systems should be designed by professional engineers in accordance with municipal standards and ministry guidelines.

22.7.4 Pretreatment of Sewage

LSSDS performance is dependent on the efficiency of the pretreatment system (e.g. septic tank, aerobic treatment process, and/or filtration), the method of sewage effluent distribution and hydraulic and organic loadings to the soil infiltrative surface, and the properties of the vadose and saturated zones underlying the infiltrative surface.

The LSSDS is mainly comprised of two components, a pretreatment process(es) (i.e., a septic tank or other treatment processes facilities) followed by a soil component (e.g. drainfield). The pretreatment facilities should be designed to achieve the treated sewage effluent quality predetermined by the hydrogeological study and soil evaluation in the drainfield area. These quality requirements may include, in addition to CBOD₅ and TSS other parameters such as pathogens, nitrogen and phosphorus.

Sewage pretreatment processes that may be considered for LSSDS include: physical and biological such as anaerobic, aerobic, and anoxic. The basic design criteria for these processes have been discussed in preceding chapters of the Guidelines.

22.7.5 Septic Tanks

The septic tank is the most commonly used anaerobic pretreatment process for a small onsite sewage system for BOD₅ and TSS reduction (50 and 70 percent, respectively) and oil/grease removal through sedimentation and flotation. The designer of a LSSDS which is to utilize septic tank(s) should follow the construction details provided in Part 8 of the Building Code. The working capacity of the septic tank(s) should be at the minimum 24-hours retention at a design peak daily flow.

The tank geometry affects the septic tank efficiency. The length-to-width (L/W) ratio, surface area and liquid depth are important considerations. Compartmentalized, elongated tanks with L/W ratios of 3:1 and greater, reduce short-circuiting across the tank and improve suspended solids removal. Other elements in the design of the septic tank that should be considered by the designer include grease/oil interceptors, inlet and outlet devices, baffles, effluent filters, access openings, gas management and other issues.

22.7.6 Aerobic Processes

Using secondary aerobic biological treatment processes (other than primary septic tanks) for lowering concentrations of BOD₅ and TSS in the effluent is recommended. By lowering organic loadings in the sewage effluent, the drainfield required area may be reduced and the life of the LSSDS system prolonged.

For flows not substantially larger than 10,000 L/d the designer should consider the use of pre-engineered (package) aerobic biological treatment units.

22.7.7 Filtration

Sand or other media may be used in packed bed filters to provide advanced treatment to septic tank or secondary aerobic treatment unit effluent in intermittent (single pass) or recirculating operation modes. Foam blocks, wood chips, peat or coarse fibre materials are used in proprietary units which may provide additional process benefits such as nitrification and denitrification. The designer should refer to the manufacturer’s specifications for further information.

Filtration of secondary treatment effluent is needed where application of a specific drainfield technology requires very high quality effluent. For a shallow buried trench method, used for soils with percolation time greater than 50 minutes, the effluent CBOD₅ concentration of 10 mg/L and TSS concentration of 10 mg/L should be consistently achieved.

22.7.8 Tertiary Treatment

The hydrogeological evaluation of the site and application of the ministry’s Guideline B-7 guideline may result in the need for reduction of pathogens, nitrogen (nitrates) and/or phosphorus concentrations in the treated sewage effluent. For guidance on nitrogen removal the designer is referred to Chapter 12 – Biological Treatment, for guidance on phosphorus removal technologies to Chapter 15 – Supplemental Treatment Processes and for guidance on sewage disinfection to Chapter 14 - Disinfection.

22.7.9 Drainfield

The minimum drainfield infiltration surface area is a function of the maximum anticipated daily effluent volume to be applied and the maximum instantaneous and daily mass loading limitations of the infiltration surface. In sizing the infiltration surface, conservative infiltration loading rates are recommended. Morphologic features of the soil, particularly structure, texture, and consistence, are better predictors of the soil’s hydraulic capacity than percolation rates. The infiltrative loading rates based on the soil morphology information shown in Table 22-1 may provide some guidance to the designers of LSSDS. The Table has two sets of loading rates; one for application of septic tank effluent (>150 mg/L BOD₅) and another set for secondary biological treatment process effluent (<30 mg/L CBOD₅).

In addition to sizing of the drainfield, the designer should give careful considerations of the placement, geometry, and depth of the infiltrative surface. Designers should consider the following for satisfactory long-term performance:

• Shallow placement of the infiltration surface;
• Trench orientation parallel to surface contours;
• Narrow trenches (width of < 900 mm [3 ft]);
• Timed dosing with peak flow storage;
• Uniform application of pretreated sewage over the infiltration surface; and
• Multiple cells to provide periodic resting, standby capacity, and space for future repairs or replacement.

The designer should be aware that the area included for calculation of dilution by infiltrating precipitation is the width of the drain field normal to the direction of groundwater flow multiplied by the length of the plume from the upgradient end of the drain field to the property boundary or surface water body. Therefore the designer should consider locating the drainfield as far upgradient of the property boundary or surface water body as possible and making the drainfield as wide as possible.

22.7.9.1 Placement of Infiltration Surface

Actual placement relative to the original soil profile at the site is determined by the desired separation distance between the infiltrative surface and the highest groundwater level, which may be below, at, or above the existing ground surface (in an in-ground trench, shallow buried, at grade, or elevated in a mound system).

Vertical separation between the infiltration surface and the water table needs to be maintained to achieve acceptable pollutant removals, sustain aerobic conditions in the subsoil, and provide an adequate hydraulic gradient across the infiltration zone. Treatment needs (performance requirements) establish the minimum separation distance, but in case of a potential for groundwater mounding, the separation distance should be appropriately increased. Effluent quality, hydraulic loading rates, temperature, soil characteristics, sewage effluent dosing pattern and distribution methods can affect the unsaturated soil depth needed for treatment. The designer should consider all these elements both separately and collectively when designing the system. Seeking reductions in vertical separation, or drainfield size based solely on the higher quality of the effluent being applied should be carefully considered, and combination of credits should be avoided. It should be noted that Part 8 of Division B of the Building Code requires 900 mm (3 ft) vertical separation between infiltration surface and a groundwater table for all code regulated systems regardless of the quality of the sewage effluent applied.

22.7.10 Depth of the Infiltration Surface

The depth of the infiltration surface is an important consideration in maintaining adequate subsoil aeration and frost protection in cold climates. The maximum depth should be limited to no more than 0.9 to 1.2 m (3 to 4 ft) below final grade to adequately re-aerate soil and satisfy the daily oxygen demand of the applied effluent.

22.7.11 Geometry, Orientation and Configuration of the Drainfield Infiltration Surface

The principles for design of large drainfields are similar to those of smaller systems for both raised and conventional beds but for large drainfields, more consideration should be given to the layout, config¬uration, and design of the drainfield. Layout and orientation will be critical as they will depend on the area which is available, the potential for groundwater mounding, the suitability of the subsurface for sewage disposal, and the impact on the ground¬water. As an example, the drainfield should be located at the upgradient side of the site to allow for the maximum amount of dilution with the underlying groundwater.

The minimum setbacks to the drainfields may need to be increased for LSSDS. The clearances from wells, surface waters and property boundaries are to be established by the hydrogeological assessment rather than the minimum values set by Part 8 of Division B of the Building Code. Drainfields and any soil mantles (if required) should be kept an appropriate distance back from the top of any slopes or from areas that are potentially unstable. Minimum depths of the mantle and minimum clearances to groundwater should be increased to minimize the potential of groundwater contamination. Through the use of manifold chambers, flow levelers, and effluent pumping systems, a maximum amount of flexibility should be developed for effluent distribution within the drainfield. This allows the operator to manually adjust the system and respond to any type of breakout that may occur. The greatest concern with large drainfields is the ability to ensure even distribution of the effluent. The design recommendation on LSSDS sites are summarized in Table 22-3.

Source: Adapted from U.S. EPA – Onsite Wastewater Treatment Systems Manual, EPA/625/R-00/008, February 2002.

22.7.12 Effluent Distribution Onto the Infiltration Surface

The size of LSSD drainfield interface surface generally precludes the use of gravity flow to the drainfields. Part 8 of Division B of the Building Code mandates effluent distribution through dosing for any sewage system having more than 150 m (490 ft) length of distribution pipe. Typically, all LSSDSs fall within this category and should be dosed appropriately.

Pumps provide the most reliable method to alternately dose the drainfields. If the drainfields are not of equal sizes or are at significantly different elevations, it is possible to use different sized pumps set on a timer system to dose the drainfields. These timers should be set to dose a specific volume to each field and generally each field would need to be serviced by a separate effluent pump. The effluent is delivered to the distribution box and gravity is used to distribute sewage effluent in the laterals.

Pressure distribution relies on fully pressurizing the distribution pipes which are typically 34 mm (1.3 in) diameter with distribution holes 8 mm (0.3 in) in diameter). The combination of full pressurization, small pipe diameter and small perforations ensure that effluent is distributed equally along the lateral.

The design should evaluate the main means of dosing:

• Demand dosing;
• Timed dosing;
• Reduced dose volume;
• Orifices in the 12 o’clock or 6 o’clock position; and
• Network remaining full or partially full between doses.

A number of advantages and disadvantages are associated with each method and should be considered when designing dosing systems. These include:

• Demand dosing is the least complex system and therefore the least costly to install and operate. However, this system is not sensitive to heavy use days and/or hydraulic surges that overload the drainfield;
• Time dosing controls the discharge to the areas with evenly spaced doses. It allows for more frequent, smaller doses to the pumped and protects the receiving component from hydraulic overload. The system is however sensitive to heavy-use days and will activate alarms when volumes exceed design levels. The system is more costly and complicated than the demand dosing system, but can also help detect groundwater leaking into the septic tank or pump chamber;
• The reduced dose volumes method produces smaller more frequent doses with intervening resting and aeration periods, which assures unsaturated flow through the soil or filter media. These systems may require smaller orifices, pipes and valves that can increase the frequency of maintenance due to clogging;
• Orifices in the 12 o’clock position will maintain the laterals full or partially full and therefore reduce the amount of effluent required to pressurize the system. Maintaining effluent in the pipes and lines can promote biological growth which will cause clogging and a buildup of solids and slime that will require more frequent maintenance. Periodic draining of the laterals can reduce clogging problems, but will increase the dose volume required. Orifices in the 6 o’clock position will reduce clogging in the laterals being drained between dose cycles, but these systems will require a larger dose volume to pressurize the system; and
• Networks remaining full or partially full between doses have the same advantages and disadvantages as indicated for the other dosing methods: that is, more frequent doses with intervening resting and aeration will promote unsaturated flow through the system, but effluent lines that are full will promote biological growth.