These systems were specially designed for small communities with very restrained budgets with respect to the operation and maintenance of their wastewater treatment facilities.
Developed for remote locations where no electricity or qualified manpower is available, and where access to the treatment facility is difficult, these systems include:
1- Trickling filters
The proposed system is composed of a primary separation tank (septic tank) followed by a first stage trickling filter (roughing filter), then by a second stage trickling filter (polishing and nitrifying) and finally a clarifying unit. The main objective of this configuration is to reduce operational costs, whilst keeping treatment at a high level.
Process Description:
Step 1: Septic Tank:
After passing through the screens, the influent will be go through the primary separation tank. The use of such tank will help reduce the particulate BOD concentration by allowing enough time for quiescent settling to occur: the expected reduction in values are 25-40 % BOD5 and 50-70% TSS. The septic tank is designed with 2 or 3 compartments to limit the discharge in the septic effluent.
Step 2: Roughing Trickling Filter
The trickling filter consists of a bed of a highly permeable medium to which microorganisms are attached and through which wastewater is percolated or trickled- hence the name. The filter media usually consist of either rock or a variety of plastic packing material. Filter beds can either come in a circular shape or a square shape, with liquid wastewater being distributed over the top of the bed by a either rotary distributor or a fixed distributor.Filters are equipped with an underdrain system to collecting the treated wastewater and any biological solids that have become detached from the media. This underdrain system is important both as a collection unit and as a porous structure trough which air can circulate. The collected liquid is to then passed to a secondary polishing unit.
Step 3: Polishing Trickling Filter
Similar to the roughing trickling filter of the first stage, this unit is larger in dimensions. All the collected liquid from the underdrain is passed to a settling tank where the solids are separated from the treated wastewater.
Step 4: Settling Tank
This tank is similar to the conventional settling system of activated sludge processes with a special care to surface loading ratio's. It is usually a square tank with hoppered bottom (45 or 60º). In order to avoid the use of a pump, a pipe with a valve is installed in the tank to remove all solids that will accumulate and transfer them to sludge drying beds.
Step 5: Sludge Drying Beds
The drying beds are of the conventional sand drying type.
Typically, the sludge is placed on a sand layer and allowed to dry. Sludge dewaters by drainage through the sludge mass and supporting sand and by evaporation from the surface exposed to the air.
2- Intermittent Sand filters
Intermittent sand filters are shallow beds of sand provided with a surface distribution system and an underdrain system. After passing by a first settling phase in the septic tank, wastewater is intermittently applied to the surface of the sand filter. The main objective of this configuration is to reduce operational costs, whilst keeping treatment at a high level.
Step 1: Septic Tank: After passing through the screens, the influent will be go through the primary separation tank. The use of such tank will help reduce the particulate BOD concentration by allowing enough time for quiescent settling to occur: the expected reduction in values are 25-40 % BOD5 and 50-70% TSS. The septic tank is designed with 2 or 3 compartments to limit the discharge in the septic effluent.
Step 2: Sand Filter
Wastewater then flows to the feeding tank from which it will be intermittently pumped by gravity to the sand filter surface through a feeding network.
Treatment of the effluent in an intermittent sand filter is brought by physical, chemical, and biological transformation. Suspended solids are removed principally by mechanical straining, straining due to chance contact, and sedimentation. Because bacteria colonize within sand grains, autofiltration caused by the growth of bacteria further enhances the removal of suspended solids. The removal of BOD and the conversion of ammonia to nitrate (nitrification) occurs under aerobic conditions by microorganism present in the sand bed. The conversion of nitrate to nitrogen gas (denitrification), routinely occurs resulting in a significant loss of nitrogen. Denitrification is brought about by anaerobic bacteria that coexist in anaerobic microenvironment within the filter bed. Specific constituent are removed by sorption (chemical and physical). To maintain a high performance level, aerobic conditions must me maintained. Intermittent application and venting of the underdrains help to maintain aerobic conditions within the filter.
