Package Treatment Plants
A package treatment plant is a premanufactured facility used to treat wastewater in small communities, mobile home parks, and summer camps, or for use by schools, businesses, or industry. They are typically extended aeration plants, sequencing batch reactors, oxidation ditches, or rotating biological compactors.
The extended aeration plant is made of steel or concrete and is a modification of the activated sludge process where air may be supplied by mechanical means or diffused aeration. Mixing by mechanical means or diffused aeration is essential for the process as well as extended contact time. For the microorganisms to work on the dissolved organics, the pH must be controlled. The remaining sludge is removed to a digester. Clarification of the effluent and disinfection occurs before disposal. This type of plant is used for small municipalities, subdivisions, apartment complexes, highway rest areas, trailer parks, etc. These units are easy to install and operate, odor free, produce very little sludge, and can handle fluctuations in the organic load and flow of the wastewater. It is best if the wastewater collection goes to the system by gravity flow. The system cannot be used for denitrification or removal of phosphorus, and requires considerable energy and a large amount of space and larger tanks than the other units.
A sequencing batch reactor is a variation of the activated sludge process where all biological treatment occurs in a single tank. The sludge is removed to a digester and the effluent is disinfected.
This type of system is used where there is little suitable land available for sewage waste processing and disposal and minimal operator attendance. It is good for industries with high BOD levels in the wastewater and requiring nitrification, denitrification, and removal of phosphorus. These units optimize the treatment efficiency and do so in a single tank which lowers the requirement for energy and space. They have few operational or maintenance problems and can be operated remotely. Sludge must be disposed of frequently and it is difficult to adjust the cycling times for a small community. Small units can have problems with operation, maintenance, and hydraulic loading.
An oxidation ditch is a modification of the activated sludge process where there is an aerated, long-term mixing of the sewage waste in a continuous manner typically in a concrete tank, although a steel and concrete tank may be used. This system can be used to treat any kind of wastewater that works by aerobic degradation and can be used for denitrification and removal of phosphorus. It provides high quality effluent with the removal of total suspended solids, BOD, and ammonia, and produces a small amount of sludge. It can be noisy, large in size, and unable to treat toxic waste. The system is expensive to operate and maintain, but it can be used if people understand its limitations.
All package treatment plants have the potential for problems caused by: a substantial and sudden temperature change; the removal efficiency of grease and scum; very small flows of wastewater; fluctuations in flow of wastewater and in the amount of BOD; hydraulic shock from overload of wastewater; and problems of the air supply.