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e) Dewatering

The two mechanical technologies most commonly used for dewatering are centrifuges and belt filter presses, with the choice being generally site specific. In all cases, pilot scale testing is recommended to optimize design.

Centrifuge dewatering is used successfully on many different types of sludges. Solid bowl centrifuges are presently used, with both countercurrent flow and concurrent flow. Newer high solids machines can produce greater than 30% solids reliably, and abrasion resistant materials prolong machine part life. Organic polymers are preferred for conditioning since they are more cost effective and have better maintenance, performance and safety records than other types of chemicals. If dewatering anaerobically digested sludge, allowance must be made for struvite formation in the centrate handling system: ferric chloride addition has been used. Electrical control interlocks with the feed system are important for efficient operation.

Belt filter presses have reduced energy requirements when compared with centrifuges and vacuum filter, but generally produce lower solids concentrations than high solids centrifuges. Polymer is used for pre-conditioning, and feed concentration should be as consistent as possible. Upstream grinders improve belt life, and rubber coatings on rollers are preferred. Most units, greater than 80%, are 1 m or 2 m belt widths.

Rotating Vacuum Filters are a technology introduced from England, consisting of a vacuum drum covered with a filter material and partially submerged in a tank of sludge. Pressure filter presses produce a drier cake than belt filter presses, can be used for a wide range of solids, and produce high filtrate quality. Partial automation makes filter presses more attractive than formerly, but they are still not as popular as vacuum filters, centrifuges and belt presses due to high capital cost, high conditioning costs, and relatively high operations and maintenance costs. Typical applications include landfill disposal and incinerator disposal where drier cakes are important. Precoating of filter using cement process dusts aids cake release and filter cleaning. A recent variation used in Toronto, Ontario, pumps hot water through the plates and pulls a vacuum to produce a very dry cake.

Rapid gravity beds are simple machines that have been used for septage dewatering. They consist of a static wedgewire screen mounted on an angle in a hydraulically operated dumping bed. The batch process uses polymer and is simple to operate for smaller communities.

Drying beds are generally used in small facilities and in the southwestern US, but can be designed for all plant sizes and for widely varying climates, although freezing can deactivate a bed during the winter months. They are more labour intensive than mechanical dewatering methods and require more land. Covered beds provide temperature and precipitation control, but open beds offer better evaporation if the weather conditions are favourable. Beds must be lined and groundwater monitoring is often required by the local authorities. Paving has been used, including heated paving in Dunedin, Florida. Other variations include vacuum assisted drying beds, and wedge-wire beds. This dewatering method can also be used as a pathogen control method in conjunction with anaerobic digestion.

Another variation on drying beds involves planting reeds in the drying beds and harvesting them annually. The method has been used in Indiana, Wisconsin, New York Pennsylvania, and Maine for plants with less than 7.5 ML/d flow. Climate should ensure at least one prolonged frost during the winter. Sludge is generally removed from the beds every 10 years.

Dewatering lagoons in warmer arid climates give the best results, but they have also been used in wetter, colder climates (i.e.Vancouver, and Winnipeg, Canada). This is a slow process, requiring 1 to 3 years.

f) Thermal drying

Sludge drying is used as a pretreatment step for composting, alkaline stabilization or incineration, or, if dried to 90 percent solids or higher, for direct land application. Dust control is important in these installations due to fire and explosion hazard, as is air pollution control equipment. Use of alternative fuels, such as digester gas, can reduce operating costs.

Direct sludge drying systems are the most common in North America. Rotary dryers have been successful at several facilities in the US and in Canada. There are approximately five flash drying installations in the US; most others have been shut down due to high energy and operations and maintenance costs, as well as fires. One fluidized bed direct dryer is operating in Mattebassett, Connecticutt.

Use of indirect dryers is limited in the U.S. There is one installation of horizontal indirect dryers at the Hyperion treatment plant in Los Angeles, and one installation on the east coast as well. There is a vertical indirect drying system at the Back Water plant in Baltimore.

Combination of direct and indirect dryers are operating at the Jones plant in Milwaukee. Their product, Milorganite, is distributed over North America as soil amendment in garden stores.

g) Incineration

Incineration is used throughout North America. It is generally economical for plants with flows greater than 38 ML/d. Fluidized bed incinerators are more popular now, and are used for plants between 38 and 76 ML/d. Multiple hearth incinerators are feasible for plants with flows larger than 76 ML/d. (WEFTEC 95, Lundberg et. al. 1995) There are seven existing infrared or electric incinerators in operation, but they are not currently being marketed for new installations due to mechanical problems and high electricity costs.

There were at least 15 fluidized bed incinerators in operation, and at least 38 multiple hearth facilities in operation in 1983. At least half of the multiple hearth installations are over 20 years old. Their age has led to air pollution control issues with new discharge regulations requiring upgrading and continuous monitoring of stack emissions.

Air pollution devices for particulate control include mechanical collectors (settling chambers, impingement separators, cyclone separators), wet scrubbers (spray tower, cyclone scrubber, ejector-venturi scrubber, venturi scrubber) and dry electrostatic precipitators. Bag houses are used if the gas temperature is reliably reduced by a heat recovery boiler.

Odour control

Control of odours from wastewater treatment plants is becoming more of an issue as communities grow to surround what were once isolated locations. Odour complaints have escalated to lawsuits against wastewater treatment plant and composting facility operators, even closing some operations. Headworks and sludge management facilities are the biggest odour sources, with fixed film processes also being a problem.

Reduction in liquid turbulence by reducing length of water drops over weirs and using submerged weirs, as well as good housekeeping, is used to control odours. Addition of chemicals such as chlorine, hydrogen peroxide, and ferric, ferrous, or zinc salts to the wastewater stream upstream of the headworks is practiced to reduce and control odours. Frequent withdrawal of sludge from settling tanks can also reduce odours from those sources. Covering odour sources such as trickling filters with aluminum or fibreglass covers and venting air to a central foul air treatment facility is also practiced.

Foul air treatment includes scrubbing with water, plant effluent, and chemicals such as potassium permanganate, sodium hypochlorite, caustic, hydrogen peroxide, chlorine and chlorine dioxide. Biofiltration using woodchips or compost, and activated carbon filtration is used. For very strong foul air streams such as from sludge heat treatment, regenerative oxidation is used.

Use of enclosed methods of conveying sludge such as screw conveyors and piston pumps rather than belt conveyors reduces odour generation. Centrifuges contain odours, while belt filter presses and gravity belt thickeners require encasement and venting of foul air to central treatment. Composting facilities are high odour generators, with the trend in new facilities being to contain the odour sources and vent to a treatment facility, generally scrubbers or biofilters.

Masking agents have been used as short term measures, but meet with mixed success and are not recommended where the odourous gas is also hazardous, i.e. hydrogen sulphide.

Release of volatile organic compounds from treatment works has become an issue in the United States.

Computer control

Central computer control systems and distributed control systems (Supervisory Control and Data Acquisition, SCADA) are being increasingly installed in wastewater treatment plants for monitoring, control and optimization of the process. The systems reduce plant staffing requirements, especially in small to medium sized plants where multiple daily shifts can be eliminated.

Distributed control systems, with a central processing unit in a central control room processing data from process control centres around the plant, and from local processing units by each process, are generally used. Remote alarms can be monitored and responded to by lap-top computer and modem. Data from on-line sensors, such as dissolved oxygen monitors in aeration basins, can be used to control equipment such as blowers. Equipment can be started and stopped using the computer control system. Redundancy is built into the control system to guard against computer downtime. Historical operating data is automatically recorded and available for analysis. Plant lab data is entered into the system. Expert systems can be built into the software as a troubleshooting tool for operators.

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