Domestic Wastewater Treatment

Domestic wastewater consists of liquid wastes that flow out after a bath, from the toilet, after washing clothes and dishes, or washing cars and other household materials. Domestic wastewater is generally divided into two types: gray water and blackwater.

The liquid waste from bathing, showering, washing dishes, and clothes is called gray water; while the water waste consisting of fecal matter and urine is called blackwater.

Gray water (also known as greywater) or sullage is considered fit for watering plants and for use in toilet flushes. If it does not have some specifically known harmful chemicals, it can be used without much worry.

Domestic wastewater treatment is focused generally on treating blackwater. Blackwater is the perfect medium for the growth of pathogenic bacteria. Therefore, it is extremely necessary to treat it before reuse or to be discharged into rivers and lakes. Sewage, another term for blackwater, is also called brown water or foul water.

If this facility is not available, it is carried away by sewers to the master plant run by the municipal authorities of the area concerned. Every city has a well-defined pipe infrastructure for this purpose.

The main purpose of the primary stage is to separate the solid and liquid matter of the sewage, so as to ease the process of treatment. This is required as there are different, specific ways to treat solid and liquid wastes.

Sedimentation is used for the purpose of separation here. Sewage is kept in large tanks designed to carry out sedimentation. These tanks are large enough for sludge to settle down at the bottom and oil, grease, and other floating material to rise up to the top, from where it is easily skimmed off.

The secondary stage is meant for the removal of dissolved biological matter from the sewage water. This task is best done by indigenous water-borne microorganisms. The solid wastes obtained by this way, is either subjected to special treatments to make it fit for reuse or is disposed of.
Secondary treatment systems are categorized as follows:

Fixed film system includes trickling filter and rotating biological contactor. These are provided with growth media to facilitate the growth of biomass and the sewage water passes through it.

In suspended growth systems, the biomass is mixed with the sewage from outside. It does not involve natural growth of the biomass. This makes it workable in a relatively smaller space and in a shorter period of time.

Still, the fixed film system has certain advantages over the suspended growth system. It is more able to manage drastic changes in the biological materials. It has higher removal rates for organic matter and suspended solids.

Activated sludge is another plant that can carry out the secondary stage. In this, air and oxygen is forced into the primary, treated sewage to provide natural conditions for the growth of biomass.

This stage is done to improve the quality of water obtained from the secondary stage of treatment. Water is treated with chemical and physical agents to remove contaminants or any undesirable mass, if present.

Sand filtration method blocks the flow of substances dissolved in water. Generally, toxins are composed of compounds of relatively smaller size. These are not filtered out by sand alone. Such smaller compounds get adsorbed on adsorbents such as activated charcoal and separate out from water. In this way, activated carbon helps in removing residual toxins.

Another very effective physical treatment method is lagooning. Water is settled in large man-made ponds or lagoons. Filter-feeding invertebrates like daphnia, macrophytes such as reeds and species of Rotifera are deliberately added to the lagoons.

Wastewater, especially domestic waste water, contains high levels of nutrients like nitrogen and phosphorus. If water is released into the environment without removing them, it may result into eutrophication.

Eutrophication is defined as the increased accumulation of nutrients such as nitrogen and phosphorous in an ecosystem. This term actually refers to excessive growth of plants, algae, and cyanobacteria and their decayed matter in the ecosystem, which becomes a cause for insufficiency of oxygen and nutrients to other living organisms in that ecosystem.

It finally results into death of ecosystem's native flora and fauna. Therefore, it is essential to remove nitrogen and phosphorus from water, before discharging it into water bodies like rivers, lakes, which are habitats for fish and other animal population.

Removal of nutrients such as nitrogen and phosphorus is done by the biological oxidation of ammonia to nitrite.(nitrification), followed by denitrification (conversion of nitrate to nitrogen gas). Nitrogen gas is released into the atmosphere and finally removed from water.

Phosphorus is the limiting factor for algal growth that gives foul smell to the water, making it unfit for drinking and domestic purposes. Phosphorus is removed with polyphosphate accumulating bacteria. They have ability to store phosphorus from the atmosphere in them. This solid mass removed has a rich phosphorus content, hence used in making fertilizers.

After wastewater has undergone all the three treatment stages, it is disinfected. Disinfection is the removal by killing or deactivation of pathogenic microorganisms from the water. It is mostly done by using chlorine, ozone, and bromine in their various forms.