Membrane bioreactors are capable of handling very high levels of mixed liquor suspended sludge (MLSS), which makes them ideal for installations with low amounts of greywater production. These plants use membrane filtration units in the place of a clarifier, which produces a much cleaner quality of treated effluent. These units are placed directly into the activated sludge and the mixed liquor is pulled through the membranes in a vacuum-driven system.
Each plant can be broken into five sections comprising of; balance tank, anoxic tank, aeration tank, sludge tank and irrigation tank. Camp wastewater is collected in the balance tank, which serves to smooth out the daily fluctuation in wastewater production. The treatment is achieved in the anoxic and aeration tanks and is based on an extended aeration process for denitrification. The liquor is constantly recirculated through these two tanks to achieve high levels of both nitrogen and phosphorous removal. The membranes are situated in the aeration tank and draw treated water out to the irrigation tank. Sludge levels must be monitored and periodically reduced by extracting sludge via the re-circulation line to a sludge holding tank, ready for the nominated form of sludge thickening or removal.
Membrane bioreactor plants are fully automated and as with most ABCO plants, require very little human interaction for successful operation. However, as with all automated equipment, a maintenance person is required to attend daily checks on the operation of the plant. Further to this, site personnel are trained in some basic preventative maintenance including chemical backwash of membranes, general plant operation and how to identify problems before they occur.
As with all of its plants, ABCO recommends a quarterly service throughout the life of the plant, to ensure reliable and consistent plant operation. This service includes checking and correcting process conditions, cleaning all probes and level sensors, inspecting pipework and tanks, and miscellaneous preventative maintenance that may be required by any optional plant equipment installed. All services are accompanied by a comprehensive service report.
Membrane bioreactors can be used in very high levels of MLSS (8 to 18%), as a secondary clarifier is not used for liquid solids separation. However, the use of submerged membranes can lead to membrane biofouling, which results in a need for regular membrane cleaning. This cleaning process typically involves a short backwash using filtered wastewater to flush the solids from the membrane surface. This cleaning process can be supplemented by a more extensive backwash which includes chemicals in the cleaning solution. Depending on the wastewater conditions, a more thorough clean is required every 6 to 12 months. This requires the removal of the membranes from service and immersion in a chemical bath to fully clean the fouling from the membranes.
These plants usually require external balance, sludge and irrigation tanks. This requirement involves a sizeable plant footprint, which again varies based on the camp capacity. A fully equipped plant servicing 1000 persons will require an area of approximately 30 square meters.