How do wastewater treatment plants (WWTPs) work?

Water is one of the most precious commodities available to humans worldwide. Water is used on a day to day basis by everyone for different purposes such as bathing, cleaning, cooking, etc. But have you ever thought about where the water ends up and what happens with the used water in the future?

Many people don’t believe this concerns them but it should as it has a detrimental effect to our environment which affects them in the long-term through global warming. Therefore, if we start contemplating for the wastewater treatment and understand how it works then you may just think twice and cut down on the time you spent in your next shower in order to save water. Before we explain the process from the start to the finish let’s first clear out a few basic definitions.

What is wastewater?

Wastewater is used water which has originated from domestic, industrial, agricultural, medical or transport industries. The used water transforms into wastewater depending upon the change of its quality, composition and/or temperature. However, wastewater doesn’t include water which is released from ponds or reservoirs for fish farming.

What are the types of wastewater?

Wastewater can be segregated into two different groups:

  1. Sewage Water is all the wastewater which is used in domestic dwellings or households which originate from toilets, sinks or showers.
  2. Industrial Wastewater is the water which originates from production, industrial and commercial activities and it has a different chemical composition compared to sewage water

In a broader perspective, we may distinguish Municipal wastewater originating from municipalities. It has a completely different ratio of sewage water, industrial wastewater and unabsorbed rain water flowing through public sewers.

How does the municipal wastewater treatment process work?

The first step is to drain the wastewater to the WWTP by gravity through the main sewar system. After that the first mechanical stage is call the preliminary treatment or rather the pre-treatment in which water flows through a gravel chamber separating out the grit from the water. The gravel which is collected is then disposed of at the dump. Water then goes further down and reaches the bar screens which are used to remove larger objects from the wastewater. At first come the coarse screens and then the fine screens which remove smaller objects such as matches, cigarette butts or undigested foods. Once the large objects are removed, then the grit is removed from the wastewater. Similar to the gravel chamber, the grit chamber allows the settlement of grit. The grit is them removed from the tank and disposed of at the dump. Neither gravel nor grit can be reused as both are highly contaminated. The next sedimentation stage is known as primary treatment in which the wastewater flows to the “pre-settling basins” also known as primary settling tanks. Over here the water is driven towards the hopper in the base of the tank. A hopper arm moves around the edge of the tank at an approximate velocity of 4cm/s. This is the point where primary pre-treatment ends and secondary wastewater treatment begins. The levels of wastewater pollution drop to 60% after the primary treatment is finished. The secondary treatment which is also called biological stage then begins. WWTPS use bacteria which consume the contaminants, in particular biodegradable organics, carbon and phosphorus. The dead bacteria and organic residue eventually transform into sludge. During the biological process stage, the excess sludge (i.e. excess bacteria) is pumped out and transported before the setting tanks. Over here, the sludge like substance settles and is moved to the digestion tanks for further treatment. In these digestion tanks, sludge is pumped out of the settling tanks to become heated and mixed. Afterwards, biogas is produced during the digestion process from the sludge which the WWTPs can reuse, for instance for thermal and electrical energy production uses. When the sludge digestion reaches optimal level, the second digestion stage takes place in the storage tanks. Over here the water is separated from the semi-solid sludge and transported back for further treatment, whereas the residual semi-solid undergoes mechanical dewatering. The sludge is then digested and dewatered to the optimal degree and is finally disposed of at the dump. After about one month, the sludge is sufficiently dried out and becomes ripe. If this final product complies with agricultural standards, then it can be reused for fertilisation of industrial crops. The last and final step of wastewater treatment is the rigorous inspection of service water. The main goal of this inspection is to analyse the contamination level and make sure that the treated water is in compliance with the highest of standards which will define its release or reuse for domestic and/or industrial purposes.

Do wastewater treatment processes exist in India?

Wastewater treatment is definitely a tough process with a significant goal which requires a lot of expertise and hard work. If you are interested in wastewater treatment or need guidance then Permionics is the best solution for you. Permionics has been the pioneer of membrane technology in India. Permionics has the unique distinction of being the first and only company in India till date to indigenously manufacture Reverse Osmosis (RO), Ultrafiltration (UF) and Nanofiltration (NF) membranes for various process specific applications and wastewater recycling, zero liquid discharge, purified water, desalination of sea water and USP purified water system.

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