Wastewater Treatment - Operation and Processes: The wastewater treatment bioprocesses transform minute solids and dissolved organic matter present in wastewaters into organic and inorganic solids that can be settled by application of flocculants.

Process analysis:

• Microbes such as bacteria – transforms particulate carbonaceous colloidal matter and dissolved organics present in wastewater into bulkier cellular lumps/tissues and into gases as a metabolic by product.
• The gases escape into the environment.
• The cellular masses are removed with the help of sedimentation tanks or clarifiers.
• The main objectives of Bio-treatments are to reduce organic matter in wastewaters mainly measured in the form of BOD, COD and TOC.
• Bio-treatments also remove nutrients (N and P) from wastewaters.
• These bioprocesses are used in tandem with other physico-chemical processes for attaining optimal effluent quality.
• Bio-processes technologies used in wastewater treatment can be broadly divided into three categories – Aerobic, Anaerobic and Anoxic.
• These processes can be run either as suspended growth system or attached growth system or as a combination of both.

Working of Conventional Wastewater Treatment Systems: The conventional treatment set up for wastewaters comprise of primary, secondary and tertiary treatments that involves various steps

• Screening is essentially to remove larger floating solids that take a very long tome for breakdown and decomposition. The screen comprises of an ordered array of flat metal plates that are welded to the horizontal bars at ~ 4 cm – 2 cm spacing. During the course of the water flow, the screens are juxtaposed perpendicular to the flow direction. The large amount of floating materials, sand debris, polymers etc stuck to the screen is removed manually or through other mechanical means. These floatable materials are then carried out as solid waste for proper disposal.
• The grit removal process mainly intends to remove heavy and inert inorganic matter. Grit, dense coarse materials, sand, shells, gravel and other heavy inorganic matter tend to settle sediments in the settling basin within a minute. The materials are then sent to proper disposal sites.
• The primary clarification happens in a settling basin that is intended for settling of heavier inorganic matter. These clarifiers have detention period of ~ 120 minutes and are mostly circular in shape. The settled materials on various parts of the clarifiers are scraped and pushed towards the centre with the help of rakers and the settled material mostly known as primary sludge are then transported to the through the primary sludge pump to the sludge digesters. Importantly in this exercise ~40 % of BOD and ~70 % of suspended solids are removed.
• Secondary treatment involving suspended aerobic processes in carried out with the help of aerobic microbes. At this stage, the wastewater are mostly devoid of particulate inorganic and organic matter and comprise of decomposed or semi-decomposed organic matter i.e. carbohydrates, proteins, lipids, fibers etc., in the presence of oxygen and aerobic bacteria these compounds are broken down into simpler forms as carbon dioxide, ammonia, water etc. The microbial activity transforms these dissolved forms into flocculating biomass and the finer organic matter into settleable mass. The oxygen is provisioned through the help of surface aerators that helps in the growth of aerobic bacteria that are required for the decomposition of organic matter. The powerful surface aerators droves the wastewater through a mechanical churning process from the bottom of the aeration tank units and splatters it over the surface thus ensuring oxygenation mobilisation.
• Secondary treatment involving attached growth processes involves of wastewater over a combination of media that acts as substrates for attachment and growth of microbes over the surfaces. In this biological process the surface grown biological microbial assembly absorbs the organic matter the wastewaters and starts multiplying of the surface of the substrates. When the weight of the surface biomass becomes critical is swept away by the trickling waters that captured in the subsequent settling units and are often recycled back. Various types of media can be used for development of the attached microbial communities as gravel, pebbles; granite of ~10-15 cm is often used in trickling filters.
• The final round of settling the solids is performed by the secondary clarifiers where the microbial flocks comprising of cellular biomass and organic aggregates are made to settle. Usually these settling clarifiers are circular in shape and with a retention time of ~90-120 min. The same rakers are used to draw the settled sludge to the centre which is then carried for recirculation to the aerobic tanks or the trickling filters. The excess amount of the solid/sludge is transferred to the sludge thickeners that separate the excess water content in the sludge. This biological process ensures ~90% of BOD removal and ~90% of SS removal of the influent wastewater.

Proposed Treatment Options for Bengaluru Sewage

Figure 9.1: Proposed wastewater treatment set-up for city sewage influx