Primary treatment removes the materials that can be easily collected from the raw wastewater and disposed of. The typical materials that are removed during primary treatment include fats, oils, and greases (also referred to as FOG), sand, gravels and rocks (also referred to as grit), larger settleable solids including human waste and floating materials. This step is done entirely with machinery, hence the name mechanical treatment.
Influx (influent) and removal of large objects
In the bar screen chamber, the influx (influent) of sewage water is strained to remove all large objects that are deposited in the sewer system, such as rags, sticks, cans, fruit, etc. This is most commonly done with a manual or automated mechanically raked screen. This type of waste is removed because it can damage or clog the equipment in the sewage.
Basically, the secondary treatment process uses all the incoming sewage as food for microorganisms. The raw sewage consumed by the microorganisms is used to provide energy for cell activity and material for cell reproduction. These processes are called respiration and synthesis, respectively.
Biological treatment effectiveness has been established for years. Municipal sewage treatment plants use biological processes in one form or another, because the process is proven and most economical.
The key feature of biological treatment processes is that the microorganisms, mostly bacteria, are able to easily remove organic wastes from a liquid that would otherwise be very difficult and expensive to remove by any other means.
There are three types of bacteria used for biological sewage treatment systems and they are all naturally contained in the sewage. One type is an anaerobic bacterium. This type of bacteria does not require oxygen for their metabolism. Some types will consume the organic waste material in the sewage, but release methane and hydrogen sulfide gas as a by-product. Both of these gases are explosive and the hydrogen sulfide produces the strongly offensive odor associated with this process. To be efficient, anaerobic processes require some form of heating.
These reasons have precluded the use of anaerobic bacteria for package type sewage treatment units.
The second type of bacteria is aerobic. They must have oxygen for their respiration and synthesis process. They consume the organic waste material in sewage and release carbon dioxide and water vapor as by-products. This does not result in any odor or explosion hazard. No special heating requirements are needed if the sewage treatment units are located where hard freezes do not occur. The aerobic process is also faster than the anaerobic process with regard to waste reduction rates.
A third type of bacteria is facultative. This type of bacteria will function as aerobic bacteria when oxygen is present, but will also function an-aerobically if there is no oxygen. Most of the bacteria population in a biological wastewater treatment plant will be composed of many different types of bacteria.
If the sewage is permitted to become stale and subsequently to become septic, its odor becomes pronounced, it turns black, the solids disintegrate and decompose. Then the dissolved oxygen is used up and the formation of hydrogen sulfide starts.
Both anaerobic and aerobic treatment systems are biological. They both depend on bacteria to consume and eliminate the organic waste material in sewage. An aerobic (air required) sewage treatment process will convert to an anaerobic (no air required) process if the air supply is cut off. It will be a gradual change, and the odor will increase as the process becomes more anaerobic. However, the process will revert back to an aerobic system one the air supply is restored. It can take 12 to 36 hours for an aerobic system to become anaerobic depending on the unit size and the amount of sewage flowing into the system.
The worldwide sewage treatment unit utilizes a type of aerobic biological treatment process call an "extended aeration activated sludge" process. This is commonly called "extended aeration" process.
After the introduction of raw sewage into the sewage treatment unit, the microorganism or bacteria call mass that is generated in the aeration chamber by cell reproduction is separated in the clarifier (second chamber) from the liquid being treated prior to discharge, collected and recycled back to the aeration (first) chamber by the skimmer and sludge return lines where it is mixed with incoming sewage. This increases the rate of removal of the organic waste coming into the system, because the waste comes into direct contact with a hungry and relatively dense population of bacteria almost immediately. This arrangement is called an "activated sludge" process.
The extended aeration treatment concept requires that the sewage treatment unit be large enough to retain and aerate the daily average flow for 24 hours in the aeration chamber. This arrangement keeps the ratio of bacteria population (sludge) to the available food supply (sewage) essentially constant. The bacteria cells are constantly multiplying, and the cell population exceeds the food supply available for feeding the excess population. These cells starve to death and become food for the survivors. The result of this process is to minimize bacteria cell sludge accumulation in the sewage treatment unit. Sewage Treatment systems that do not employ the extended aeration treatment concept will require considerable drain off and disposal of sludge, because bacteria are not kept away from a food supply long enough to keep the bacteria population from increasing. As a result, the bacteria population will continue to increase to a point where the sludge density becomes so great that the sludge begins to disperse and carry over with the effluent from the sewage treatment unit
Secondary treatment is designed to substantially degrade the biological content of the sewage such as are derived from human waste, food waste, soaps and detergent. The majority of municipal and industrial plants treat the settled sewage liquor using aerobic biological processes. For this to be effective, the biota requires both oxygen and a substrate on which to live. There are number of ways in which this is done. In all these methods, the bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc. Secondary treatment systems are classified as fixed film or suspended growth. Fixed-film treatment process includs trickling filter and rotating biological contactors where the biomass grows on media and the sewage passes over its surface. In suspended growth systems—such as activated sludge—the biomass is well mixed with the sewage and can be operated in a smaller space than fixed-film systems that treat the same amount of water. However, fixed-film systems are more able to cope with drastic changes in the amount of biological material and can provide higher removal rates for organic material and suspended solids than suspended growth systems.
Activated sludge is a process dealing with the treatment of sewage and industrial wastewaters. In general, activated sludge plants encompass a variety of mechanisms and processes that use dissolved oxygen to promote the growth of biological floc that substantially removes organic material.
The process traps particulate material and can, under ideal conditions, convert ammonia to nitrite and nitrate and ultimately to nitrogen gas, (see also denitrification).
Most biological oxidation processes for treating industrial wastewaters have in common the use of oxygen (or air) and microbial action. The basins may range in depth from 1.5 to 5.0 metres and utilize motor-driven aerators floating on the surface of the wastewater.
In an aerated basin system, the aerators provide two functions: they transfer air into the basins required by the biological oxidation reactions, and they provide the mixing required for dispersing the air and for contacting the reactants (that is, oxygen, wastewater and microbes). Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kg O2/kWh. However, they do not provide as good mixing as is normally achieved in activated sludge systems and therefore aerated basins do not achieve the same performance level as activated sludge units.
Biological oxidation processes are sensitive to temperature and, between 0 °C and 40 °C, the rate of biological reactions increase with temperature. Most surface aerated vessels operate at between 4 °C and 32 °C.
Typical Criteria for designing of STP’s
- Flow in cum/day
- Inlet BOD in mg/l
- Inlet TSS in mg/l
- Outlet BOD required in mg/l