Co-treatment of septage and landfill leachate at sewage treatment plants

Chapter 19: Co-Treatment of Septage and Landfill Leachate at Sewage Treatment Plants

This chapter includes design considerations for treatment of septage, landfill leachate and other wastes hauled to municipal sewage treatment plants. Introducing these hauled waste loadings into sewage works for co-treatment with sewage exerts demands on the processes that are disproportionate to typical sewage hydraulic and organic loadings. Smaller sewage treatment plants in particular should be aware, before accepting such loadings, that a single load may overload and potentially upset their processes and may cause effluent quality deterioration.

19.1 Septage

19.1.1 General

One method of septage disposal and treatment is discharge to a municipal sewage treatment plant (STP).

The term “septage” for the purpose of these guidelines refers to the contents removed from septic tanks, portable toilets, privy vaults and holding tanks serving houses, schools, motels, mobile home parks, campgrounds and small commercial endeavors, all receiving sewage from domestic sources. Although septic tank waste may be the highest in concentration of this group and potentially more stabilized than other holding tank type wastes, these wastes are generally combined in the load discharged at the plant and therefore they are often considered as a group. In cases where the hauled waste is solely holding tank waste, the characteristics would be similar to regular domestic sewage.

19.1.2 Characteristics

Compared to raw domestic sewage from a conventional municipal sewage collection system, septage usually has high concentrations of organics, grease, nutrients, hair, stringy material, scum, grit, solids and other extraneous debris. Substantial quantities of phosphorus, Total Kjeldahl Nitrogen (TKN), total ammonia-nitrogen (TAN), bacterial growth inhibitors and cleaning materials may be present in septage depending on the source, typical concentrations are provided in Table 19-1. A comparison of some of the common parameters for septage and municipal sewage is given in Table 19-2.

Data for local septage to be received should be collected for design of septage receiving and treatment facilities. The characteristics of septage should be expected to vary widely from load to load depending on the source. If co-treated at an STP, treatment of septage will increase the amount of solids generated.

Table 19-1 - Physical and Chemical Characteristics of Septage

Parameter	Average Concentration (mg/L)	Minimum Concentration (mg/L)	Maximum Concentration (mg/L)	EPA Mean1	Suggested Design Value1, 2
TS	34,100	1,100	130,500	38,800	40,000
TVS	23,100	400	71,400	25,300	25,000
TSS	12,900	300	93,400	13,000	15,000
VSS	9,000	100	51,500	8,700	10,000
BOD5	6,500	400	78,600	5,000	7,000
COD	31,900	1,500	703,000	42,800	15,000
TKN	600	100	1,100	700	700
TAN	100	5	120	160	150
Total P	200	20	760	250	250
Alkalinity	1000	500	4,200	-	1,000
Grease	5,600	200	23,400	9,100	8,000
pH	-	1.5	12.6	6.9	6.0
Linear Alkyl Sulphonate (LAS)	-	110	200	160	150

Table 19-2 - Comparison of Contaminant Concentrations in Septage and Sewage

Parameter	Septage1 (mg/L)	Sewage (mg/L)	Ratio of Septage to Sewage
TS	40,000	720	55:1
TVS	25,000	360	69:1
TSS	15,000	210	71:1
VSS	10,000	160	62:1
BOD5	7,000	190	190
COD	15,000	430	35:1
TKN	700	40	17:1
TAN	150	25	6:1
Total P	250	7	36:1
Alkalinity	1,000	90	11:1
Grease	8,000	90	89:1
pH	6.0	-	-
Linear Alkyl Sulphonates (LAS)	150	-	-

The general characteristics for septage that may be hauled to an STP are discussed below. The designer should evaluate the potential impacts of these characteristics and assess whether the plant has enough capacity and proper equipment to co-treat septage effectively considering the large variability in the physical-chemical characteristics of septage. Due to the contents and nature of solids in septage, their impact on existing grit removal equipment, pumps and air diffusers should be specifically evaluated.

In many respects, septage is similar to domestic sewage, except that septage is significantly more concentrated. Differences in contaminant concentrations between septage and domestic sewage are outlined below:

BOD₅

The BOD₅ of septage can be as much as 30 to 50 times or more concentrated than that of normal domestic sewage.

TSS

Compared to domestic sewage, septage can be very high in total suspended solids (e.g. 10 to 50 times typical STP influent total suspended solids). Evaluation of solids characteristics of local septage waste is recommended and should include total solids (TS), total suspended solids (TSS), total volatile solids (TVS) and settleable solids (SS).

Fats, Oils and Grease

Almost no decomposition of grease occurs at a sewage treatment works and the expense of handling and disposing grease can be considerable. Contents from grease traps should not be hauled to municipal STP for treatment. Rendering and other recycling options are often available and preferable to handling such matter at a sewage treatment works.

Grit

A household septic tank will accumulate grit, rocks and other dense material in its sediment layer over the years. After cleaning out many septic tanks, the accumulation of this sediment load in the septage hauling tank can be several hundred kilograms. Because of this concern for downstream sedimentation, discharge into a sewage collection system should be avoided. The septage receiving station at the STP site should have provisions for an adequate rock sump. Even with an adequate rock sump, dense grit can form a compacted layer in a sewer after several years of routine septage discharge into the collection system.

Odour

Due to the anaerobic nature of a septic tank system and the mixture of organic materials, septage is very odorous. Design should include means to control these potential sources of odour.

Nutrients

The concentrations of nitrogen and phosphorus in septage are high compared to typical domestic sewage and needs to be accounted for. This is especially important if nitrification and/or biological nutrient removal is required at the STP.

Heavy Metals

Metals in septage may come from household chemicals, leaching of plumbing pipes and fixtures and possible contamination from previous industrial loads hauled in the septage hauling truck. Because metals do not decompose and the interval between septic tank pumpings can be several years, metals tend to accumulate in septage.

19.1.3 Treatment

Septage is normally considered treatable at STP when proper engineering design is provided. The designer should consider the following factors to minimize shock loadings or other adverse impacts on plant processes and effluent quality:

• Hydraulic capacity [m³/d (mUSgd)] of the plant relative to the volume and rate of septage directed to the plant;
• Unused plant capacity available (above current sewage collection system loadings) to treat septage loadings;
• Sensitivity of the treatment plant process to daily fluctuations in loadings brought about by the addition of septage (e.g. slug loads);
• Slug septage loadings of BOD₅, TKN, TAN or phosphorus which may cause process upset (e.g. to the nitrification process, if applicable), odour nuisance, aeration tank/aerated digester foaming or pass through to the STP effluent;
• The point of introduction of the septage into the STP process. Feasible alternative points of feed to the treatment units should be evaluated including feed to the sludge processing units provided the unit function will not be adversely affected. Generally septage can be introduced at the following locations within a plant:
  • Upstream of the primary clarifier;
  • Directly into an anaerobic digester but with caution not to overload the digester [maximum Total Volatile Solids (TVS) loading should be controlled];
  • At small extended aeration plants, septage can be added to an aerobic digester;
• Blending septage with sludge prior to dewatering should not be considered;
• The ability to control feed rates of septage to the plant for off-peak loading periods;
• The volume and concentrations of bacterial growth inhibitors in septage from some portable vault toilets and recreational dump station holding tanks; and
• Treatment of septage in a sewage stabilization pond (lagoon) can be done but requires evaluation depending on the number of stages (lagoons in series) and the amount of septage received compared to sewage flow and may require equalization of septage. TAN removal may be a problem due to high loads from the septage and additional treatment such as intermittent sand filtration may be required following lagoon treatment. (Section 12.3.6 – Intermittent Sand Filters)

The effluent quality requirements on each of the controlled parameters should be considered when evaluating these factors.

If the septage is primarily septic tank solids, which are partially stabilized, this material may be successfully introduced into the sludge treatment process (i.e., digesters) after pretreatment. Loadings need to be carefully reviewed, potential impacts on sludge treatment processes (e.g. dewatering) assessed and equalization considered. Since septage in Ontario is often a combination of septic tank and holding tank wastes, it is generally first treated through the STP liquid train.

19.1.4 Design Criteria

The decision to treat septage flows as a part of the conventional municipal sewage treatment process has several significant effects. Treating these flows increases the load on both the liquid and solids trains of the STP with resulting increases in operating cost, solids production, solids handling and disposal or utilization costs. Accepting this loading consumes a greater proportion of the capacity than similar volumes of normal sanitary sewage flow. Treating septage flows can affect the ongoing operation and the quantity of biosolids produced at a given facility.

Water Environment Federation (WEF) Septage Handling - Manual of Practice No. 24 (1997) and other references provide ranges of design values. Although literature values for BOD₅ and other septage constituent concentrations are available, assessment of the actual septage composition and volumes that are expected locally should provide the basis for design.

Design of the STP processes should account for septage loading as a part of the complete design. Loading assumptions and design criteria for septage receiving should be calculated separately in addition to the main influent sewage loading assumptions.

19.1.5 Considerations

In the case of existing STP, it is essential that an adequate engineering evaluation be made of the existing processes and the anticipated septage loading prior to receiving septage at the plant. For proposed plant expansion and upgrading, the engineering report needs to include anticipated septage loading in addressing treatment plant sizing and process selection (Chapter 2 - Project Design Documentation). The following items should be included as appropriate in the engineering evaluation:

• The continuous and satisfactory treatment (i.e., continuously meeting the plant effluent quality criteria) of sewage loads from the sewer system should not be adversely affected by the addition of septage to the plant for co-treatment;
• The smaller the plant design capacity relative to the septage loading, the more susceptible the plant will be to process upset and potential violation of effluent quality requirements;
• Allocation of plant organics treatment capacity originally planned for future growth;
• For plants to be expanded and upgraded, the engineering evaluation should include the sensitivity of the treatment process(es) to receiving septage and the impact on the effluent quality. Amendments need to be made to the Certificate of Approval of the sewage works to account for new septage receiving facilities;
• An evaluation of available plant operator staff and the staffing requirements necessary when septage is to be received;
• The space for constructing septage receiving facilities that is to be off-line from the raw sewage incoming from the sewer system. The location of the septage receiving facility and the septage hauler unloading area should consider other plant activity and traffic flow;
• The amount of additional sludge generated will depend on the type of treatment used, the septage-to-sewage ratio and septage characteristics; and
• The impact of the septage handling and treatment on the plant sludge handling and processing units and ultimate sludge disposal procedures.

Several software packages are now available for simulating models of sewage treatment processes and full-plant models. Based on a calibrated model of the existing facility under current operating conditions, estimates of the effects of increased organic or hydraulic loadings (e.g. associated with septage additions) can be made.

19.1.6 Receiving Facility

The design of the septage receiving station at the sewage treatment plant should provide for the following elements:

• A hard surface haul truck unloading ramp sloped to a drain to allow ready cleaning of any spillage and washing of the haul tank, connector hoses and fittings. The ramp drainage should be tributary to treatment facilities and should exclude excessive stormwater;
• A flexible hose fitted with easy connect coupling to provide for direct connection from the haul truck outlet to minimize spillage and help control odours;
• Electronic metering and billing systems are available to monitor septage received and provide accurate billing information to septage haulers and plant staff. These systems generally consist of a card reader or key pad for controlled access in combination with a flow meter and valve;
• Washdown water with ample pressure, hose and spray nozzle for convenient cleaning of the septage receiving station and haul trucks. The use of chlorinated effluent may be considered for this purpose;
• An adequate off-line septage receiving tank should be provided. The tank should be sized to hold twice the maximum daily volume of septage expected on a peak day. Capability to collect a representative sample of any truck load of waste accepted for co-treatment at the plant should be provided. The receiving tank should be designed to provide complete draining and cleaning by means of a sloped bottom equipped with a drain sump. The design should give consideration to adequate mixing. Adequate mixing will ensure uniformity of septage strength and mixing for chemical addition, if necessary, for treatability and odour control;
• Screening, grit and grease removal or grinding of the septage as appropriate to protect the STP process units;
• Pumps provided for handling the septage should be of the non-clogging design and capable of passing 100 mm (4 in) diameter solids;
• Glass-lined pipes are recommended;
• Valving and piping for operational flexibility to allow the control of the flow rate and point of septage discharge to the STP; and
• Safety features to protect the operational personnel.

19.2 Landfill Leachate

Landfill leachate is produced as a result of rain percolating through the landfill waste and reacting with the products of decomposition, chemicals and other materials in the waste. Typically, landfill leachate is anoxic, acidic, rich in organic acid groups, TKN, TAN, sulphate and chloride ions and with high concentrations of common metal ions especially iron.

Acceptance of leachate for co-treatment at an STP should be assessed and then carried out with the same precautions outlined in the guidelines provided above for septage. Due to the extreme variability of leachate, dependent on its source, it should be reviewed on a case-by-case basis to determine its properties. Unlike septage, leachate can contain components of industrial nature and a more complex sampling program may be necessary. Potential inhibitory effects of leachate constituents and loadings on STP processes (e.g. on nitrification, if applicable) need to be assessed.

Pretreatment of landfill leachate should be considered to reduce the strength and equalize the loading to the STP. A variety of biological and chemical treatment options can be used to pretreat leachate on-site. For biological treatment, characterization and treatability tests should be conducted to ensure the leachate on its own is treatable without additional nutrients and chemical adjustments.

19.3 Other Types of Hauled Waste

Other wastes may be hauled to the sewage treatment plant, but these like all septage and hauled wastes from industrial and commercial sources should be characterized and expected loads and delivery times known. The capacity and capability of the STP to co-treat these waste types need to be assessed on a case-by-case basis. This will ensure that the impact of these wastes on the STP operations can be determined before being accepted. It is inappropriate for hazardous and flammable wastes to be co-treated at STP and are excluded.

19.3.1 Chemical Toilet Waste

Materials from portable toilet facilities are commonly called chemical toilet waste. Portable toilets are pumped similarly to septic tanks and transported to a treatment works for co-treatment. Commonly a chemical is added to the portable toilet’s holding tank to control odours. Characteristics of chemical toilet waste need to be assessed for potential impacts on the STP.

19.3.2 Recreational Vehicle Waste

The characteristics of recreational vehicle (RV) waste are similar to chemical toilet waste.

19.3.3 Vactor Waste

Many sanitary sewer collection systems use vacuum maintenance equipment to clean sewer lines, catchbasins, manholes and pump station wet wells. Depending upon the source, the resulting composition of the vactor load can vary widely. A full vactor truck may contain materials from several different types of cleaning assignments. Any vactor spoils contaminated with sewage should be properly treated and disposed. If vactor wastes are received from sources other than sanitary sewers, these wastes need to be characterized before being accepted.

19.3.4 Waste from Other Sewage Treatment Plants

Sludge or sewage received from other sewage treatment facilities should be assessed on a case-by-case basis.

19.3.5 Water from Soil Remediation

Water byproduct from soil remediation processes (mostly groundwater) should be assessed before discharging to municipal sewage treatment plants.