Control technologies and management practices are used in water treatment plants to improve the prevention, treatment, disposal, and discharge of water treatment residuals. This helps treatment plants to meet standards, improve water quality, reduce treatment system operation costs and provide energy savings. Pollution prevention (e.g., process modifications) and waste reduction (e.g., resource recovery) opportunities in water treatment plants are the preliminary steps in residual management, which aims the reduction of the generation of residuals. Optimizing intake water conditions, filter media, pH to reduce coagulant chemicals, reducing softening chemicals by monitoring source water hardness, returning backwash water and filter-to-waste to the head of the source water treatment plant for reuse, reusing precipitative softening chemicals, recovering treatment chemicals coagulants and salt can provide waste reduction and help residual management. Unavoidable residuals from the water treatment operations can be treated in water treatment plants prior to final waste management such as land application, disposal and discharge. The treatment options for residual management in water treatment plants are;
The principle objective of solids removal systems is decreasing the volume of water while increasing solids content. Enough dry solids percentage depends on desired properties for handling, transport and disposal options available. Thickening/dewatering is often for this purpose. The objective of thickening is to increase the solids content of the residuals. This is important in the reduction of the capital and operating costs of the continuing treatment. The most common thickening technologies are gravity settling, dissolved air flotation, and gravity belt (Table 8.1)In gravity settling, solids are separated (thickened) from water by using gravity within the designed residence times. In dissolved air flotation, which is the most common of several flotation separation technologies, the floating material is the thickened solid and skimmed off. If space is limited in water treatment plant area, or if gravity settling or flotation do not provide the desired solids thickening, then gravity belt thickener can be used as an alternative. They are simple designs and require minimal operator but they generate another residuals stream (wash water) and usually require use of a solids conditioner and maintenance. Supernatant from the thickening operation is recycled or discharged. The thickening operation is followed by mechanical dewatering, the next solid/water separation step. Mechanical dewatering is used for additional volume reduction and concentration of solids. Since source water, site, and other conditions vary from facility to facility, the systems, needs, and constraints should be thoroughly assessed in order to select the mechanical dewatering equipment that best suits the utility's specific requirements. Selecting the right equipment will ensure optimum performance. Belt filter presses, plate and frame filter presses, and centrifuges are commonly used. For all mechanical dewatering systems pre-conditioning is generally required. Mechanical forces are applied during dewatering, thus, some water is released from the sludge depending on the characteristic of the sludge. Each mechanical dewatering device applies forces in different ways; therefore, the resulting cake dryness varies. These technical systems require a high degree of operator supervision and training. They are rarely cost efficient for small systems.In belt presses feed solids usually are introduced into a feed box via pumping and distributed across the dewatering surface under atmospheric pressure. As water drains through the pores, a thin layer of cake is formed which builds up as the sludge is moved into the shear zone and pressure zone.Pressure filters (e.g., plate and frame filter press, diaphragm filter press) the sludge is pumped at high pressure in between the filter plates and force the liquid out while retaining the solids.Centrifugal dewatering is a process that centrifugal force is applies to ensure separation of solids from liquid. Centrifuges have some advantages, like; its being totally enclosed, continuous operation, having relative ease of operation, high throughput and cake solids, high solids capture, and being compact and having space-saving design.Following collection and thickening, the sludge can be further concentrated or dewatered by non-mechanical dewatering methods, as well. Lagooning, drying on sand beds, natural or artificial freezing and thawing (physical method), and chemical conditioning are typical non-mechanical sludge dewatering methods.In lagoons and storage ponds the residuals are spread out, exposed to the air and allowed to dry by evaporation. Application of this method depends on the land availability, evaporation rates and ground water contamination concerns.An improvement over the sludge lagoon is the sludge drying bed which incorporates a permeable medium and a system of under drainage. The climate and land availability affect the choice of this method. Where the application is possible, sand drying beds are effective and relatively inexpensive method of dewatering water treatment plant residuals. Sand drying beds (gravity drainage+evaporation), freeze-assisted sand beds (allowed to freeze and then thaw either naturally or mechanically; mostly used for alum residuals), vacuum-assisted systems (applying pressure to speed up the drying process) and solar drying beds (where climate is sufficient) are the non-thermal drying bed technologies. Thermal drying technology is generally used to solve problems with pathogen control, odour control, and storage problems and is not widely used. Coagulant sludge usually requires solids removal systems. It usually has a 0.5 to 2.0 present solids concentration. Coagulant sludge can contain high percentage of gelatinous, hydroxide precipitates depending on the total suspended solids concentration of the source water. Softening sludge is easier to dewater and compact than coagulation sludge. Softening sludges are generally dense, stable, and inert materials.
Table 8.1. Comparison of Solids Removal Technologies: Solids Concentration After Treatment by Residuals Type
| Solids Removal Treatment | Solids Concentration for Treated Lime Softening Residuals | Solids Concentration for Treated Coagulation Residuals |
|---|---|---|
| Thickening | ||
| Gravity Thickening | 15–30% |
1–3% (low TSS) 5–30% (high TSS) |
| Flotation Thickening | Not available | 2–4% |
| Gravity Belt | Not available | 2.5–4.5% |
| Mechanical Dewatering | ||
| Scroll Centrifuge | 55–65% | 20–30% |
| Belt Filter Press | 50–60% |
1 –20% (Alum) 4–50% (Alum, TSS) |
| Plate (or Pressure) Filter | 55–70% | 35–45% |
| Diaphragm Filter Press | 50–70% | 30–60% (Alum with lime conditioning) |
| Non-Mechanical Dewatering | ||
| Storage Lagoon | 50–60% | 7–15% |
| Sand Drying Bed | 50% | 20–25% |
Source: U.S. EPA, ASCE, and AWWA, 1996.
Chemical precipitation is used to remove dissolved metals from residuals by the addition of a precipitating reagent. This treatment is applicable to liquid waste streams, generated in filter ion exchange and membrane desalination concentrates. Mostly hydroxide is used as precipitating reagent. Lime, quicklime, soda ash, or caustic soda can be added to introduce the hydroxide ions. Sulphide and ferrous salt can also be used depending on the metals present in the residuals. Chemical precipitation of the residuals is commonly used to remove aluminium, antimony, arsenic, cadmium, chromium, copper, iron, lead, mercury, selenium, silver, thallium, or zinc removal.
Generally, oxygen is added to residuals prior to discharge in order to control biological oxygen demand discharges and increase dissolved oxygen levels.
After disinfection, free or total combined chlorine residual remaining can be observed which is toxic to aquatic life. This is removed by dechlorination through the addition of optimum dose of sulphur chemicals like sulphur dioxide, sodium sulphite, sodium bisulfide, sodium metabisulfide, and sodium thiosulfate.
Chemical additions in treatment operations to improve treatment performance, alter the pH of source water is altered during treatment. To adjust the pH to range between 6 and 9, acids or bases are added.
