Sewage is the liquid waste from toilets, baths, showers, kitchens, etc. that is disposed of via sewers. In many areas sewage also includes some liquid waste from industry and commerce. In many countries, the waste from toilets is termed foul waste, the waste from items such as basins, baths, kitchens is termed sullage water, and the industrial and commercial waste is termed trade waste. The division of household water drains into greywater and blackwater is becoming more common in the developed world, with greywater being permitted to be used for watering plants or recycled for flushing toilets. Much sewage also includes some surface water from roofs or hard-standing areas. Municipal wastewater therefore includes residential, commercial, and industrial liquid waste discharges, and may include storm water runoff.
Sewage treatment is the process that removes the majority of the contaminants from waste water or sewage and produces both a liquid effluent suitable for disposal to the natural environment and a sludge. To be effective, sewage must be conveyed to a treatment plant by appropriate pipes and infrastructure and the process itself must be subject to regulation and controls. Other wastewaters require often different and sometimes specialized treatment methods.
The Sewage is collected in a tank, called sewage sump or sewage collection tank. In case septic tank already exists, it can be used as sewage sump. The sewage from sump will then constantly pumped into Domestic sewage treatment plant (DSTP) by employing sewage lift pumps at a constant flow rate. Piping arrangement is also made to re-circulate the effluent to enable the contents of the sewage collection tank be in a mixing state. DSTP is of modular construction and comprises of bar screen, collection tank, Aeration section (SAFF REACTOR), Sedimentation Section(Tube Settler), Treated Sewage Holding/Transfer Chamber and Chlorination.
A further consideration made in treatment process design is the removal of primary clarifier. Primary clarifier has deliberately not been used in the process because of following two reasons. Firstly, the problem of sludge separation, handling and its disposal. The sludge generated from the primary clarifier will be highly biodegradable & if not treated properly would cause severe odor problems due to natural anaerobic degradation taking place at the dumping / storage area. The treatment of the sludge either by anaerobic digestion or by aerobic digestion will be very costly affair. On the other hand the excess sludge if produced from the secondary system will be well oxidized & stabilized so that it can be disposed off as a land- fill to act as soil enriching bio-fertilizer.
This being a biological system, SAFF reactor to be operated continuously all 24hr and thereby constant feed of sewage & air is required. The coarse screen of 100 mm is provided before the collection tank to trap the floating debris, if any and then pumped to the aeration tank through a fine screen of 50mm and grit channel. In the aeration tank, the biomass is oxidized to CO2 and H2O by the aeration principal. Aeration tank (SAFF) is fitted with fixed extended surface media and air diffusers of suitable capacity to provide necessary oxygen for bio-degradation of sewage organic.
As aeration tank design is based on extended aeration principal and minimal sludge is produced. The sedimentation tank/tube settler, which is designed on low overflow rate is provided after the aeration tank to enable separation of solids. A steep slop is provided in the settling tank to eliminate the need of scrapper mechanism. The settled sludge being collected in the bottom of tube setter is partially re-circulated back to SAFF reactor to give a constant feed to the bacteria & balance is taken to the sludge drying beds to dry & disposed off.
The clarified water collected from the collection launder of tube settler is then passed through a chlorine contact tank, where shot dosing of chlorine is given & further through pressure sand filter & activated carbon filter. The treated sewage shall have the following characteristics.
|Oil & Grease||<10mg/1|
Considering the raw sewage load as follow.
|Oil & Grease||<10mg/1|
|BOD-5days at 20 degree||300 mg/It|
|Oil & Grease||<50 mg/It|
It is expected that the treated effluent be used for land irrigation or discharged to drain.
Effluent from various sections shall be drained under gravity to a screen chamber for removal of floating materials & transferred to oil & grease trap for removal of oil and grease coming from the unit. Effluent is further taken to underground basin called equalization tank equipped with oil free air stripping/ purging system.
The air induced effluent is pumped to the two stage isolated reactor where in dosing of selective flocculants is imparted. The flocculated effluent is let into primary tube settler for separation of flocks. The primary tube settler is provided with special type of media, which helps in faster settling of flocks. Standard rate of settling in conventional clarifiers with scrapper mechanism is 18m3/m2/day whereas rate of setting with special media is 3.5 m3/m2/hr. The supernatant from the primary tube settler is taken to SAFF Reactor (Submerged Aerobic Fixed Film Reactor).
SAFF Reactor is provided with Fixed media in blocks. The size of every block is 1.2m x 0.6m x 0.6m(H) & volume of each block is 0.432 m3. SAFF reactor is provided with silicon rubber Tubular diffusers for diffused aeration. The supernatant from SAFF reactor is taken to secondary tube settler for separation of activated sludge from the treated effluent. The activated sludge from the secondary tube settler is partially recycled back to the SAFF reactor & rest of the sludge is taken to the sludge drying beds. The clarified water from the secondary tube settler is collected in a sump & further passed through pressure sand filter & activated carbon filter for separation of refractory elements & suspended impurities.
Sludge from tube settlers is withdrawn on sludge drying beds. Sludge on drying disposed off by way of land filing. The treated effluent is with aforesaid characteristics may be reused for dyeing after mixing a part of fresh water. A common twin lobe air blower is provided with air diffusion system for equalization for effluent, SAFF reactor & for Chemical Mixing tank.
The treated effluent shall have the following characteristics
|Oil & Grease||<5 ppm|
|Oil & Grease||1500 ppm|
|BOD-5days at 20 degree||300 mg/It|
|Oil & Fats||20 ppm|