Handling of Chemicals
Chapter 20: Handling of Chemicals
This chapter outlines the handling of chemicals that are commonly used at sewage treatment plants (e.g. for phosphorus removal, disinfection, and sludge thickening and dewatering).
20.1 General
Chemicals selected for use in sewage treatment plants (STP) should not adversely affect the operation of the sewage or sludge treatment processes and should not leave dangerous or harmful residuals in the effluent or sludge leaving the plant. The purity of chemicals proposed to be used should be determined. Occasionally, waste streams from industry, such as ferrous chemicals, can be used at STP provided that they are not contaminated with other hazardous materials.
Laboratory, pilot- or full-scale studies of various chemicals, feed points and applicable treatment processes are recommended for STP to determine the achievable performance level, cost-effective design criteria and ranges of chemical dosage requirements.
The selection of appropriate chemicals and dosages for STP should be based on such factors as influent sewage characteristics, the proposed chemical, effluent quality requirements and applicable treatment process requirements.
Common uses of chemicals and hazardous materials at sewage treatment facilities include sewage treatment, process enhancement and control, housekeeping, landscaping, laboratory, maintenance, fuel supply and odour control. Common chemicals and hazardous materials used in STP include, but are not limited to, those shown Table 20-1.
20.2 Storage of Chemicals
Storage should be provided for at least thirty days of consumption at the maximum anticipated chemical usage rate, allowing for variations in sewage flow and chemical dosage in that period. Where deliveries of chemicals may be interrupted by adverse weather conditions in isolated locations, provision should be made for increased storage capacity taking into consideration that some chemicals degrade with time (e.g. sodium hypochlorite solution). Where deliveries can be assured on short notice and the material is not essential, storage requirements can be reduced.
| Treatment Chemical | Combustible, Hazardous, Flammables and Explosive Hazards |
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Some of the considerations that will have an effect on the chemical storage volume requirements are as follows:
- Chemical usage rate and delivery time;
- Typical volume delivered (storage should be at least one truck load plus 25 percent);
- Availability of alternate suppliers;
- Loss in strength of chemical with storage time;
- Seasonal changes in chemical strength; and
- Critical nature of chemical with respect to treatment process.
Structures, rooms and areas accommodating chemical storage and feeding equipment should be arranged to provide convenient access for chemical deliveries, equipment servicing and repair, and observation of operation. It is recommended that wherever possible the storage area be separated from the main plant and that segregated storage be provided for each chemical. Where two or more chemicals could react with undesirable effects, the drainage piping (if provided) from the separate chemical handling areas should not be interconnected. For dangerous materials such as gaseous chlorine, either floor drains in the storage and scale rooms should be omitted entirely, or floor drains installed, but kept totally separated from the drainage systems for the rest of the building.
Chemical storage areas need to be enclosed in dykes or curbs that are capable of containing 110 percent of the maximum stored volume until it can be safely transferred to alternate storage or released to the sewage at a controlled rate that will not damage facilities, inhibit treatment or contribute to receiver pollution. Curbs, sloped areas and drains should be used to contain spills at unloading areas.
It is strongly recommended that all chemical storage be at or above the surrounding grade. Where subsurface locations for chemical storage tanks are proposed, these locations should be free from sources of possible contamination, and positive drainage (i.e., pumped) for ground or storm waters, chemical spills and overflows needs to be assured. Where above grade storage is provided, due consideration should be given to the method of unloading chemicals; for example, there is a limit on the allowable pressures to be used for air-padded trucks. Where dry chemicals are used, it is recommended that some form of loading dock or ramp be provided.
The storage areas and locations should be arranged to prevent any chemical spills and to facilitate clean-up operations. The floor surfaces should be smooth, impervious, slip-proof and adequately sloped to drainage points.
Adequate measures should be taken to provide a minimum temperature of 15 °C (59 °F) for chlorine gas areas and the remainder of the chemical buildings should be heated to a temperature to prevent crystallization or freezing of chemical or abnormally high viscosities (polyelectrolytes) which would make pumping difficult.
The ventilation system for chemical buildings should be such that exhausted air is passed outside the building and arranged within the building to provide for slight negative pressures in areas where dry chemicals are in use, as a dust control measure. Where large amounts of dust are anticipated from chemical handling operations, adequate air filtration equipment should be included in the ventilation system.
The vents of tanks containing chemicals that are sensitive to moisture should be equipped with desiccant cartridges.
The designer should note that special precautions may be necessary in the design of air emissions control systems to prevent chemical concentrations at the point of impingement from exceeding limits permitted within the building or site by Air Pollution-Local Air Quality, (O. Reg. 419/05) made under the Environmental Protection Act (EPA) or which might be hazardous. An approval under Section 9 of the EPA is required if a contaminant may be discharged into the air from any part of the plant. For details, see the ministry’s Guide for Applying for Approval (Air and Noise).
All chemical buildings should be provided with eye wash units and/or deluge showers, adequate facilities for cleaning up chemical spills, space for cleaning and storage of the recommended protective equipment and adequate warning signs, conspicuously displayed where identifiable hazards exist. It is recommended that all doors in chemical rooms open outward and that corridors or space between storage areas be a minimum 1.5 m (5 ft) wide to permit the use of equipment such as hand trucks.
The use, storage and handling of any hazardous materials should be in accordance with federal and provincial legislation (e.g. Regulation 860, Workplace Hazardous Materials Information System (WHMIS), made under the Occupational Health and Safety Act as well as the Building Code, O. Reg. 350/06 made under the Building Code Act, 1992 and the Fire Code, O. Reg. 388/97 made under the Fire Protection and Prevention Act, 1997).
Chemical buildings or storage areas should be provided with adequate warning signs, conspicuously displayed where identifiable hazards exist, a storage area for Material Safety Data Sheets (MSDS) and other provisions in accordance with the Occupational Health and Safety Act (OHSA), R.S.O. 1990, c O.1. All storage containers should be conspicuously labelled with a WHMIS label that includes: the product name, the supplier name, hazard symbol(s), risk, precautionary measures and first aid measures. An MSDS should be available for each chemical.
20.2.1 Liquid Chemicals
All storage tanks should be constructed of a material proven for the intended service. Since some chemicals, such as ferric chloride, are delivered at very high temperatures (up to 60 °C or 140 °F), the tanks and associated equipment should be able to withstand such temperatures. Tanks located outside should be heat traced and insulated to maintain the minimum allowable temperature for the chemical being stored.
All storage tanks should be provided with an adequate size fill line, minimum 50 mm (2 in) in diameter, which is sloped to drain into the tank. The fill line should be adequately identified at the end remote from the tank and provision should be made to drain this fill line, if a “down leg” exists.
Each tank should have an adequate vent line, minimum size 50 mm (2 in), with a down-turned end. Where venting outside the room is required, the vent should be provided with an insect screen.
All tanks should have an overflow adequate for the rate of fill proposed for the tank and overflows should be sloped down from the tanks, with ends turned down and having insect screens, and should have a readily visible free discharge directed to a suitable containment area. Tanks to be filled by pumping should be provided with an overflow not less than 300 mm (12 in) above the design level and not less than 150 mm (6 in) above the design level when filling is done by gravity flow.
Each tank should be provided with means to indicate the level of the contents in the tank and where an external level gauge is provided, it should have a shut-off valve at the tank connection. Each tank should be equipped with a drain. Tanks should have removable lids or covers or manholes where the contents are such that venting indoors is permitted. In the case of tanks which are to be vented outside, the covers or manholes should be constructed so as to be airtight. Overflows from tanks holding corrosive chemicals should be provided with seals to prevent vapours migrating to the room.
Tanks should be arranged to provide a minimum clear space all around them of not less than 300 mm (12 in). Where tanks with liners are used, weep holes should be provided in the outer shell to show positive indication of liner leakage.
A containment system should surround liquid storage tanks to contain spills, having a capacity exceeding the volume of all storage vessels contained (i.e., 110percent of overall content volume).
All storage tanks should be conspicuously signed with the contents and principal hazards of the contents shown.
20.2.2 Dry Chemicals
Where dry chemicals are to be used, provision should be made to minimize dust problems in handling. The use of granular materials is preferred.
Particular care should be taken with fine dusts around electrical equipment. Where exhaust fans, filters and vacuum conveying systems are used, grounding should be provided to prevent any static electricity build up.
20.2.3 Liquid-Gas Chlorine
Gas chlorination equipment (chlorinators, weight scales, chlorine cylinders) needs to be located in an isolated room or rooms. In larger installations the storage and weighing facilities should be in a room separate from the chlorinators. The construction of the facility should be of fire resistant and corrosion-proof material, have concrete floors and be gastight. All interior surfaces should be coated with a substance impermeable to chlorine gas.
Safety chains need to be used to retain each cylinder, in storage and on weigh scales, in a safe upright position. Chlorine should not be stored below ground level and the cylinders need to be protected from excessive heat, dampness and mechanical damage. One-tonne cylinders should be stored on their sides on level racks.
Areas containing chlorine or chlorination equipment need to be clearly marked. The exit doors with panic hardware needs to be hinged to open outwardly. There needs to be two or more exits if the distance to travel to the nearest exit exceeds 5 m (15 ft). All exits from the chlorine room and storage area should be to an outside wall. Access between these rooms is permitted if they have a common wall.
The temperature in the chlorine storage and scale room should not be higher and preferably slightly lower than that in the chlorinator room. The gas lines between the scales, chlorinators and injectors should not be located directly on an outside wall or in a location where low temperatures may be encountered.
As previously indicated, floor drains from chlorine storage or scale rooms should not be connected with drainage systems of other parts of the building or other buildings. Chlorine gas is heavier than air and could travel via drains, such as floor drains and foundation drains, into other rooms. If floor drains are to be used, they should be completely separated from other drainage systems. As an alternative to floor drains, the floors may be sloped towards doors to provide the needed drainage.
Gas detectors and alarms should be provided for storage and scale rooms. If the plant is not continuously manned or connected to a plant Supervisory Control And Data Acquisition (SCADA) system, the alarm should terminate at a fire station, police station or other 24-hour manned location.
Each sewage treatment works using liquid-gas chlorine should have a contingency plan to deal with major gas leaks.
Chlorine gas feed and storage rooms should be provided with inspection windows to permit viewing of the interior of the room and equipment. Switches for fans and light should be outside the room at the entrance and a signal light indicating fan operation should be provided at each entrance. Vents from feeders and storage should discharge to the outside atmosphere above grade and should slope down wherever possible.
20.3 Chemical Application Points
All chemicals should be applied to the sewage streams or sludges at such points and in such a way as to ensure the maximum efficiency for treatment and to provide maximum safety to the operators. Chemicals should be added at a point of turbulence or at a point of mechanical or induced mixing or through a diffuser to ensure satisfactory mixing. Particular care should be taken where the point of addition is close to a point where the flows split. Alternate chemical addition points should be provided to give maximum flexibility of operation where appropriate. Where chemicals are added to lines under pressure, a suitable isolating valve should be provided.
With phosphorus removal chemicals, it is desirable to terminate the chemical feed point above the liquid level of the tank or channel so that the chemical discharge can be observed by the operator.
20.4 Chemical Feed Equipment
Where the chemical added is necessary for the protection of the receiving waters such as chlorination, dechlorination, phosphorus removal or other critical processes, a minimum of two feeders should be provided, with one acting as a standby unit.
The design and capacity of feeders should be such that they will be able to supply at all times, the necessary amounts of chemicals at an accurate rate, throughout the feed range. Feeders should be capable of proportioning the chemical feed to the rate of sewage or sludge flow.
Chemical solutions can be prevented from being siphoned into the sewage stream by either assuring discharge at a point of positive pressure or providing vacuum relief or a suitable air gap.
All positive displacement pumps should be equipped with adequately sized pressure relief valves. If the pumped fluid is relieved through this valve, it needs to pass to a safe location, preferably back to the storage tank. Where liquid-filled diaphragm pumps are in use, the over-pressure should be relieved internally or by discharge of the motive fluid to a safe location. Pressure relief valves should be set at a safe relief pressure to avoid damage to the chemical feed lines.
Chemical feed lines should be kept as short as possible, protected against freezing and readily accessible through their entire length.
The minimum line size should be 12 mm (0.5 in), unless the material pumped exhibits scale forming tendencies, then the minimum size should be 25 mm (1 in). In general, the feed line should be designed to be consistent with the scale forming or solids depositing properties of the material conveyed. Where feed lines are provided from duplicate pumping units or passed to a distribution manifold, adequate valving should be provided to isolate sections of the line.
Where reciprocating type pumps are to be used, it is recommended that flexible connections be provided on the pump suction and discharge to prevent the transmission of vibrations to the feed line. These flexible connections should be sufficiently rigid to withstand both the pump suction and discharge pressure, and reinforced hose is recommended. The pump, in combination with its suction piping and valving arrangement, should be such that the pump discharge rate remains the same regardless of the level of chemical in the feed tank.
Where dry chemical feeders and storage equipment are provided, the design of the storage equipment should be adequate to prevent bridging or other problems in the storage silo. Dry chemical feeders will be acceptable if they measure either volumetrically or gravimetrically and provide for effective solution of chemical in a solution pot. The dry chemical feed system should be enclosed to prevent emission of chemical dusts into the operating room.
Feeders may either be manually or automatically controlled and automatic control should revert to manual control as necessary. Feed rates proportional to flows should be provided.
Where solution tanks are to be used, the designer should provide as means to maintain a uniform strength solution. Continuous agitation should be provided to maintain slurries in suspension. Normally, two solution tanks will be required to ensure continuity of supply in servicing the solution tank. Each tank should be provided with a drain.
Make-up water for the solution tank should enter the tank not less than 150 mm (6 in) above or two pipe diameters above the maximum solution level, whichever is greater.
Where the design of the chemical feed system includes day tanks, such day tanks should have a maximum capacity equivalent to the chemical consumed over a 30-hour period. Day tanks should either be scale-mounted or have a calibrated level gauge provided. The piping arrangement for refilling the day tanks should be such that it will prevent over-filling of the tank. In all other respects, the requirements for day tanks should conform to the requirements for bulk storage tanks.
20.5 Operator Safety
The design of sewage works need to include provisions to protect operator and other worker safety and health. The safety of workers and workplaces is governed by the Occupational Health and Safety Act and the Workplace Safety and Insurance Act, as well as the regulations made under these acts, which are both administered by the Ontario Ministry of Labour.