Both naturally occurring and artificial media have been used to treat wastewater in small community settings. Systems using naturally occurring media include sand filters and peatbased biofilters. Artificial media include open cell foam, textiles, and plastics. All the above operate as fixed-film systems, in that attached growth microorganisms are encouraged to colonize and reproduce in the media. Fixed film systems appear to produce a more consistent effluent under peaked sewage inputs and periods of no use (vacation homes, weekend retreats) compared to suspended growth systems in this flow range. Naturally occurring media have advantages in that naturally occurring fauna and flora may already be present in the media. Start up is therefore shortened because colonists are already present in the media and just need to multiply to handle the waste renovation process. A disadvantage of naturally occurring media is that they must be mined. Extraction of these natural resources is sometimes criticized because they formed over thousands of years. They will eventually be exhausted and will not be replaced. Quanics. Inc. has patented a product that combines advantages of both artificial and naturally occurring media. The Bio-COIR™ filter uses coir, the recycled husks of coconut in a biological fixed film filter. The material is a waste product of agricultural operations in developing countries. The product has successfully passed NSF Standard 40 certification. Systems in the ground for two years show virtually no change in media properties except a slightly darker color, giving the expectation that the media will continue to function for a comparable period to peat (8 to 16 years between media replacement).
A variety of media have been used to treat wastewater. In the small community sector, both naturally occurring and artificial media have been employed. Systems based on naturally occurring media include sand filters and peat-based biofilters. Artificial media include open cell foam, textiles, plastics, and recycled, ground glass (Stuth and Garrison, 1995).
All such filters operate as fixed-film systems, in that attached growth microorganisms are encouraged to colonize and reproduce in media. Under these circumstances, wastewater must be brought to the microorganisms living in the media under aerated conditions and applied at light enough dosages to encourage effluent flow via thin films throughout the media surfaces. Because attached microorganisms cannot move to areas with sufficient food or free oxygen, these necessities of life must be brought to them. Because water is denser than air, the energy requirements for attached growth systems are therefore higher than for suspended growth systems.
There are other differences between suspended growth and attached growth systems. The reader is directed to Sherman, 2006 for details on some of these. In this paper, the major distinction to be highlighted is system performance under peaked loading conditions. In suspended growth systems, the microorganisms responsible for treatment float freely in the aeration chamber. Under a regimen of consistent sewage inputs, stable populations develop that can rapidly aerobically stabilize wastewater (Tchobanoglous, 1991). However, in gravity-fed systems, if sewage loads temporarily exceed anticipated values, the unit may ‘burp’ activated sludge out of its clarifier. This situation is not cause for alarm if the unit is connected to a standard drainfield. The high dissolved oxygen in the effluent will soon reach the escaped microbial cells and allow their complete aerobic digestion. The author has dug into over a dozen drainfields associated with functioning suspended growth Aerobic Treatment Units of various manufacturers and has yet to encounter a restricting layer or biomat. However, the number of microbial cells, or activated sludge, left in the aerator has been diminished (Hutzler, Waldrop and Fancy, 1978).
Gravity-fed Suspended Growth ATUs are most common in single-family home settings. They operate under the hydraulic displacement principle. In simple terms, when one gallon flows into the unit, the effluent level inside the unit rises until one gallon exits. Consequently, hydraulic detention time in the aeration and clarifier chambers is variable. Under unanticipated high loading (e.g., fifty gallons in), not only have some microbial cells left the clarifier because their settling velocity was less than the exit velocity up and out of the clarifier, the time that a parcel of water has spent in the aeration chamber is shorter than usual. CBOD5 and TSS in the effluent will therefore temporarily be outside of accepted values (Bennett, Linstedt and Felton, 1975).
The very efficiency of aerobic microorganisms works against them under periods of low or no sewage inputs. Now the activated sludge microorganisms are not being fed enough. Because these microorganisms are free-floating they are able to encounter each other and are able to eat each other. The microbial populations under extended periods of under feeding changes to a ‘pin floc’ condition. In this state the few remaining microorganisms are in torpor. When fresh sewage is again added to the unit, there will be a lag time until the microorganism population size and vitality are restored.
Because of the above, when aerobic treatment is desired the attached growth process is indicated for intermittent and extended high or no flow situations. Examples of establishments where these flow patterns occur are vacation homes, weekend retreats, churches, flea markets and rental properties.
The start up period for natural media filters is shorter than for artificial media filters. This is because natural media already contain low numbers of microorganisms, flora and fauna. Their reproduction provides the biomass needed for sewage treatment (Brooks, Rock and Strochtemeyer, 1984). In artificial media the assumption is that the organisms responsible for renovating wastewater must first colonize, then reproduce in the media for effective treatment.