Energy efficiency and Reuse of STP Treated water

Abstract

Sewage generated from household need to be treated before it enters into any water body. The importance of water treatment and supply is not given to sewage though it is about 80% quantity of water supply. To achieve the SDG goals of water supply and sanitation and also life below water, it is essential to treat the sewage and also reuse after treatment to reduce the overall demand of fresh water being used. One of the main causes is sewage treatment plants are power intrinsic and requires lot of funding for operation and maintenance of STPs. The old STPs in operation are consuming more power and need of the hour is to renovate and refurbish the plants with latest developments in the field. Improving the energy efficiency of the plant need to be studied and implemented for making the STPs viable for operation.

The sewage generated will vary with the time of consumption during the day. The peak flow which is 2 to 3 times the average flow, average flow and lean flow which is half of the average flow. Peak flow exists for 5 to 6 hours in a day. Average flow exists for 6 hours a day and lean flow exists for 12 to 13 hours in a day. Sewage coming from under ground drainage network or open nallas will reach the treatment plant premises at a depth of minimum 5 to 6 m below natural ground level which necessitates pumping for further treatment . Sewage pumping is almost 40% of the power consumed in treatment scheme. Number of measures are to be taken for decreasing power consumption. A) using number of small size pumps to decrease the power consumption and also use suitable pumps based on the flow regime during the day B) Using variable frequency drives for optimal power consumption C) Use of level sensors and SCADA controlled operation for timely starting and stopping of the pumps for using minimum energy

Another power consuming component of treatment plant is aeration. Dissolved Oxygen levels are to be continuously monitored and the oxygen requirements are to be modulated using variable frequency drives for the blowers. By using online oxygen uptake rate as a tool for monitoring and controlling the status of the plant during aeration phase of the treatment plant. Oxygen requirements vary as per the inlet BOD and oxidation need to be adjusted in the process. Using turbo blowers in the system will increase the efficiency and less power consumption. Twin lobe , Tri lobe blowers can also be used with proper preventive maintenance coupled with VFD for adjusting the oxygen transfer efficiency. Online monitoring and using SCADA for knowing the real time DO levels will reduce the power consumption by about 30%. The efficiency of the selected blowers will be high at design parameters like discharge and head which will  vary during the cycle time of aeration and also as per the level in the reactor. It is very crucial to supply oxygen as per the treatment phase and as per input BOD levels.

Secondary treatment plants can be converted into tertiary treatment units by providing gravity disc filters there by reducing the BOD less than 5 mg/lit which make the water fit for reuse of Indirect potable use. This water can be used for industrial applications like cooling, recirculation and other non process water. This will ensure to meet the SDG goals efficiently. By providing number of case studies where the water reuse has been effective and economically viable, the use of the same for many more applications will increase.

Waste to wealth concept can be discussed further by promoting the bi-products from STP meeting the sustainable Development goals.