OTHER PRODUCTS
Aluminium Sulfate
Coagulation
During coagulation, a coagulant such as aluminum sulfate or iron salt such as ferric sulfate or ferric chloride added to raw water and mixed in the rapid mix chamber destabilizes negatively charged particles, or dissolved, and colloidal contaminants. Coagulant aid polymers and/or acid may also be added to enhance the coagulation process. Simply put, coagulant dosing generally depends on turbidity and TOC concentrations in the raw water. Generally, higher TOC and/or higher turbidity levels require higher coagulant dosages.
Particle destabilization generally includes charge neutralization and sweep flocculation depending on the coagulation conditions. In charge neutralization, attraction of the positively charged metal coagulant to the negatively charged particles produces charge neutral particles that can agglomerate as particle collisions occur. These charge neutral particles can then be removed through physical processes.
Many water treatment plants operate using sweep flocculation, which requires a higher coagulant dose when compared to charge neutralization (American Water Works Association, 2011). Excess coagulant beyond the charge neutralization dosage results in the formation of metal coagulant precipitates (e.g., Al(OH)3 or Fe(OH)3), which are heavy, sticky, and larger in particle size. Sweep flocculation occurs when colloidal contaminants are entrained by the precipitates and settle in the suspension.
For downstream filtration processes, namely granular media filtration, to work properly the floc particles approaching the filter must have a near-neutral charge because the filter media generally has a negative surface charge and would therefore repel the negatively charged particles. Zeta potential measurements can be used to directly measure particle charge in colloidal suspensions by measuring the electrical potential near the surface of a solid particle. As zeta potential approaches zero, interparticle repulsive forces decrease, and particles can agglomerate into larger floc particles that are more amenable to clarification and filtration. The zeta potential at which good water treatment performance occurs varies for each water source, but the American Society of Testing and Materials recommends ±5 mV, although some facilities have demonstrated successful treatment at values ranging from –10 to +5 mV (Pernitsky et al., 2011).
For high alkalinity water, excess coagulant can be added to lower the pH to the optimal pH range, assuming the coagulants are acidic. In some cases, acid can be added to the water with the coagulant to reduce pH to the optimum level, which reduces coagulant usage, solids production, and treatment costs. Enhanced coagulation, now widely practiced for removing disinfection by-product (DBP) precursors, can also more effectively remove inorganics, particulates, and color-causing compounds. Additional information related to DBP control is presented in Chapter 1.7.
Coagulant dosing should be assessed, often through jar testing, to ensure proper coagulation conditions for effective treatment. Underdosing coagulant will not effectively destabilize the particles, whereas excess dosing can result in restabilization or excessive sludge production. The pH during coagulation controls the coagulant speciation and solubility and can affect contaminant speciation. Temperature also impacts the coagulation process because it affects the viscosity of the water and solubility of the coagulant (American Water Works Association, 1999). Thus, lower temperature waters can decrease the hydrolysis and precipitation kinetics. For some treatment objectives, other parameters like iron, manganese, or sulfate impact coagulation.
Coagulation requires high-energy mixing, often called rapid mixing, between the raw water and coagulant to provide the initial contact between the water and chemicals. This provides the appropriate conditions for coagulation, which causes the destabilization of particles and allows flocs to develop during flocculation.
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Waste Water Treatment
Sanitation and Sustainable Development in Japan is a joint publication by the Sustainable Development and Climate Change Department (SDCC) of the Asian Development Bank and Japan Sanitation Consortium (JSC).
On-site wastewater treatment poses a challenging problem for engineers. It requires a balance of appropriate levels of technology and the operational complexity necessary to obtain high-quality effluent together with adequate reliability and simplicity to accommodate infrequent maintenance and monitoring. This services covers how these issues have been addressed in on-site wastewater treatment in Japan (termed johkasou). On-site systems in Japan range from outmoded designs that discharge grey water directly into the environment to advanced treatment units in high-density areas that produce reclaimed water on-site. Japan is a world leader in membrane technologies that have led to the development of on-site wastewater treatment units capable of water-reclamation quality effluent. Alternative ideas being pursued for on-site technologies also include separate waste stream collection, which would provide for more efficient treatment and reuse. Night soil treatment plants, where sludge from on-site systems is treated, are also distinctive to Japan, serving 37 million people. Japan has governmental regulations in place to ensure routine inspections of on-site units; furthermore, subsidies are available to reduce the cost of on-site systems for building owners. Lessons learned in on-site wastewater treatment in Japan have applications worldwide, from regions where water is scarce, to high-density areas in developing countries that currently lack sewer infrastructures.
Solid Waste Management and Recycling Technology of Japan
Due to economic development, industrialisation and increasing population, problems related to the expanded consumption and depletion of resources, and the increased output of wide-ranging types of waste are becoming more serious than ever. There is a word in Japan: Mottainai. It encompasses the practice of treasuring and using all things as long as possible. While economies continue to grow, this spirit of Mottainai restrained the generation of waste and motivated the development of technology for reuse, recycling and effective use through heat recovery. As Japan's landmass is limited and finding landfill disposal sites is difficult, we have developed a system to collect and transport waste, process it through intermediary treatment by incineration and other methods, and then dispose it in landfills in a sanitary manner, in order to prevent environmental pollution in the areas surrounding densely populated cities.