More than one of the treatment processes listed below are used in water treatment plants and may generate multiple types of residuals;
Pre-sedimentation is the first unit process that is operated at a water treatment plant. Gravity removes suspended solids from source in pre-sedimentation. In the pre-sedimentation process, the efficiency of solids separation and removal depends on the residence time. Sand and grit settle more rapidly than clay, silt and organics. Depending on the composition, 50 to 90 percent of the influent solids can be removed in pre-sedimentation. However; the composition of the solids in the sludge is site-specific. The smaller particles are removed during the coagulation, flocculation, sedimentation, and filtration processes.
Chemical agent(s) (generally aluminium and iron salts) are added to reduce the negative surface charges of smaller particles by introducing positive ions in order to allow the particulates to agglomerate and settle to the bottom of clarifiers in coagulation, flocculation, sedimentation process. The added chemical agent(s) precipitate along with the neutralized suspended solids by gravity. The volume of sludge generated in this process depends on the treatment plant capacity, amount of coagulant or other treatment chemical added, and amount of suspended solids in the source water whereas the characteristics of sludge alter depending on the quality of initial water and the amount and type of coagulant used. For example; higher aluminium concentration in the sludge is expected when aluminium-based coagulant is used. “Coagulation sludge mainly contains the coagulant metal hydroxides along with source water natural organic matter, suspended solids, microorganisms, radionuclides, and other organic and inorganic constituents. The metals found in coagulation sludge include aluminium, arsenic, and occasionally cadmium, chromium, copper, iron, lead, manganese, nickel, and zinc (Cornwell, 1999).”
Precipitative softeninig is used to remove divalent ions, mainly calcium and magnesium in water in water treatment plants, by the addition of lime. When lime is added to water, it increases the pH of it and it reacts with the ions to form a precipitate. The precipitate contains calcium carbonate, magnesium hydroxide, other divalent ions, natural organic matter from the source water, inorganics, suspended solids, microorganisms, and radionuclides with a solids content of 2 to 15 percent. It is inert and it has a pH typically higher than 10.5. Arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver can also be found in the softening sludges. The sludge generation rates in lime softening vary according to the ratio of calcium carbonate to magnesium hydroxide and the type of sedimentation tank.
Finer particles and metals are removed in water treatment plants by filtration process. At some water treatment plants, filtration is the only solids removal step. In water treatment plants; there exist non-membrane filters such as multimedia, slow sand and diatomaceous earth. Besides low-pressure membranes such as microfiltration (MF) and ultrafiltration (UF) can also be used.Non-membrane filters remove suspended material in water resources. While the water passes through the filter media, suspended solids accumulate in the interstices. As the water runs through the pores and more particles accumulate, the filter performance decreases. At this moment, the filter is taken out of service for backwash which is the process of using clean water to reversely expel the particles collected on the filter media. Clay and silt particles, microorganisms, colloidal and precipitated humic substances, natural organic particulates and precipitates of aluminium or iron used in coagulation are present in filter backwash waters. The number of filters, frequency of backwash, and duration of backwash events affect the volumeof filter backwash wastewater generated. The volume is typically between 2 and 5 percent of the finished water produced (U.S. EPA/ASCE/AWWA, 1996). Flow equalization systems are generally designed to recycle the backwash water to the head of the water treatment plant. After backwashing, filters are washed in order to gain adequate filter performance. The spent wash water, which is called “filter-to-waste” is the filter effluent for the first 15 to 60 minutes after start-up following the backwash. The filter-to-waste stream is equalized and returned to the head of the treatment plant.Filter backwash waste streams are also generated by low-pressure membranes. Low pressure membrane systems are also used to remove suspended solids, turbidity, inorganic and organic colloids, microorganisms; and ultrafiltration is used to remove viruses and some organic compounds. 95 to 99 percent of the total volume of residuals is generated by MF/UF treatment process. To remove some solids prior to the membrane; coagulants, powdered activated carbon (PAC), or other chemicals (e.g., potassium permanganate) as pre-treatment to membrane filtration can be used in some systems. This reduces the backwash frequency. The resulting residuals generally show the characteristics of coagulation sludge. During the membrane cleaning processes, spent chemical cleaning solution residuals are also generated. The chemical cleaning waste includes some remaining active chemical ingredient, and some salts from chemical reactions between the chemicals and foulants, dissolved organic materials, and suspended solids.
In water treatment plants, membranes are generally used to remove dissolved solids and ions. They are also used to remove dissolved organics, dissolved gases, biological contaminants, and suspended solids. Reverse osmosis (RO), nanofiltration (NF), electrodialysis (ED), and electrodialysis reversal (EDR) are membrane desalination technologies. In order to protect and extend the life of the membrane, the source water prior to membrane desalination is generally pre-treated. In this pre-treatment; to lower pH to between 5.5 and 7.0 acid addition, to prevent membrane fouling anti-scalant addition and to remove suspended particles filtration take place. Backwash waste stream is generated in this filtration step. In membrane desalination systems generated contaminants are concentrated on the membranes for separate disposal. The contaminants characteristics are similar to those of source water, however, if pre-treatment is applied, of ‘course lower levels of certain constituents and particles but higher levels of constituents added in pre-treatment process are expected. For instance, in case of coagulation pre-treatment, inorganic ions, such as sulphate, iron, and aluminium, and polymer is observed and sulphuric acid pre-treatment may increase residual organics.
Ion exchange is used to reduce hardness of source water. Sodium ions that are contained in the ion exchange resin are replaced with calcium and magnesium ions in water. Nitrates, barium, radium, arsenate, selenite, excess levels of fluoride, lead, and chromate can also be removed in ion exchange unit. The ion exchange material is regenerated when the site capacity is reached. This regeneration produces waste concentrate that contains the source watercontaminants. Besides, backwash water and rinse water that is used before and after the regeneration of the ion exchange resin, respectively is generated in ion exchange process, as well.
Adsorption is used to remove organic materials, taste, odour, synthetic organic compounds and disinfection by-products. Granular activated carbon is the most common adsorptive media in adsorption process. In activated carbon adsorption, ions or molecules in the water is removed by enabling them adsorbed onto the treatment media. Backwash water and spent media are generated as residuals. As the treatment process goes on, when the pollutants are no longer removed from the influent because of the filling of adsorption site (breakthrough), backwashing of the filter bed should be performed to disengage solids that have been entrapped in the filter bed. Breakthrough occurrence depends on the concentration of the pollutant contaminants being removed. Backwash water typically contains the removed contaminants and some granular activated carbon. The volume and quantity of the granular activated carbon backwash stream depends on the influent source water quality. The spent carbon is regenerated or disposed. Regeneration of the spent carbon is done by thermal means and does not generate a wastewater stream. Common pollutants found in water treatment plant residuals have potential environmental impacts on the receiving environment therefore; they should be handled within the water treatment plant. Suspended solids for example; may settle in the receiving water and form bottom deposits, creating anaerobic conditions; increase turbidity and decrease light penetration in receiving waters, thus limit the growth of plantation that serves as habitat for aquatic organisms; provide a medium for the transport of other sorbed pollutants; clog fish gills and alter the chemistry of natural waters. Metals are potentially toxic to aquatic organisms, including fish and have the potential for bioaccumulation and biomagnification in aquatic food chains. Chemicals used for disinfection like chlorine and chloramines can react with organic compounds in receiving waters to form toxic compounds. Exceeded chloride levels in wastewater discharges can disrupt ecosystem structure and impair their use as source waters for potable water supplies. Sudden pH changes can kill aquatic life in receiving water bodies. Ammonium and ammonia nitrogen is toxic to aquatic life and can reduce dissolved oxygen concentration that is vital for microbial activity. Together with phosphorus, they are the limiting nutrients and cause eutrophication. Exposure to radionuclides increases the risk of cancer.
