Dewatering in its simplest definition is the removal of water. This process is used in many industries but commonly referred to in construction and wastewater when water is separated from solids through a variety of different pumping or filtering processes. Construction dewatering is often referred to as dewatering, unwatering, or water control. It involves pumping from wells or sumps to temporarily lower groundwater levels, to allow excavations to be made in dry and stable conditions below natural groundwater level.
In wastewater treatment, dewatering is the part of the process whereby sludges are reduced in volume and converted from a liquid to a solid product. Biosolids dewatering typically occurs when transportation and storage costs for large volumes can be reduced or when the material is destined for landfill. The biosolids dewatering process not only effects the volume but also the nutrient and odour levels of the material.
Dewatering Techniques
- Centrifuge: The centrifuge works in a similar nature to a front loading washing machine. The spinning action causes a separation of water from the solids. This process typically requires a large power input and polymer addition. The system works best with a consistent slurry or feed sludge and provides a dewatered product between 16-35% solids.
- Belt press: If a centrifuge can be compared to a front loading washing machine then the belt press can be compared to a wringer on an old washing machine. The method of separation is primarily obtained by passing a pair of filtering cloths or belts through a system of rollers. The system takes a sludge or slurry as a feed, and separates it into a filtrate and a dewatered product between 12-35% solids.
- Geo-textile: High strength permeable fabrics are woven into dewatering bags that can be filled with slurry. The water permeates from the dewatering bag through the small pores in the geo-textiles resulting in effective dewatering and volume reduction of the contained solid material. Although somewhat slower than mechanical dewatering options, geo-textile dewatering is an excellent dewatering option, reducing costs, and energy inputs. This method can produce material from 15-45% solids.
- Rotary Vacuum: This method of dewatering involves the suction of liquid through a filter. Because the filter itself can be changed depending on the project needs, the solids capture rate is very high. Material can be filtered down to 0.5 micron producing unparalleled effluent quality. This process while slower than other mechanical dewatering options provides material with 20-45% solids.
Dewatering and Waste Management
Dewatering is used by large wastewater treatment plants to separate sludge into a liquid and solid. The principle methods in wastewater are belt filter presses and centrifuges.These systems are high maintenance and require a high degree of supervision and operator training. They are usually only implemented at larger facilities and are not cost efficient to be used on a small scale. This is only one part of the process of wastewater becoming treated water and biosolids. Primary treatment is essential prior to the dewatering. The filtrate or centrate liquid which is separated during the dewatering process must also be treated. This typically involves circulation to the headworks of the wastewater treatment plant.