Generally wastewater treatment involves the following processes:
1. Preliminary Treatment:
The velocity of the wastewater from the sewer lines is reduced as it enters the treatment plant. This allows sand, gravel and other heavy materials to settle out into grit tanks. Mechanical cleaning bar screens then remove rags, sticks, plastic and other foreign objects from the wastewater; this part of the treatment plant is called the head-works.
Bar screens may be used before or after the grit tanks. All removed material collected by grit tanks and bar screens is washed and then taken to a sanitary landfill for disposal.
2. Primary Treatment:
Primary treatment allows for the physical separation of solids and grease from the wastewater, and removes between 30-40 per cent of Biological Oxygen Demand and 50 per cent of Total Suspended Solids (TSS).
The screened wastewater flows into a primary settling tank where it is held for several hours allowing solid particles to settle to the bottom of the tank. Fats, oil and grease (FOG) are skimmed from the tanks, dried and sent to the landfill.
The settled particles are known as primary sludge, which is collected and pumped to large digestion or holding tanks for further treatment and solids processing.
3. Secondary Treatment:
Secondary treatment is a biological treatment process that removes upto 90 per cent of BOD and TSS. Following primary treatment, effluent is pumped to the secondary treatment stage. It may take one of several forms; for example, either a trickling filter or sequencing batch reactor. The growth of micro- orgisms results from the consumption of organic matter in the wastewater as their food supply.
The microorganisms create a solid organic material (sludge), which just like the sludge from primary treatment. Secondary sludge is thickened and pumped to digesters for processing and solids processing.
4. Final Treatment:
The wastewater that remains can be disinfected to kill harmful micro-organisms before being released into receiving waters. Although there are many methods available to kill these microorganisms, ultraviolet disinfection, ozonetion and chlorination are few of them. At this stage, the treated wastewater, now called final effluent, is discharged into the marine environment.
5. Solids Processing:
Primary solids from the primary settling tank and secondary solids from the clarifier are sent to digesters for solids processing. During this process, micro-organisms use the organic material present in the solids as a food source and convert it to by-products such as methane gas and water.
Digestion results in a 90% reduction of pathogens and the production of a wet soil-like material called “biosolids” that contain 95-97% water. In order to remove some of this water, mechanical equipment such as a belt filter press or centrifuge are used to squeeze water from the biosolids, reducing its volume prior to being used in soil amendment or composting.