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EHA : Yearkbook 2009
EHA YEARBOOK 2009 60 consistently performing were the OzziKleen and the sand filter systems which met both parameters (BOD5 and SS). It is interesting to note that both the OzziKleen and the Sandfilter systems had the lowest mean age after the Enviro- Cycle. Table 4 provides the compliance rate for the models tested for suspended solids. Overall System Performance Figure 3 outlines the overall performance in terms of system compliance with the EPA Victoria effluent standards for both BOD and SS. Note that if one system did not meet one of the parameters for BOD5 or SS, then it was deemed to have not met the standard overall. This shows that four (27%) systems met both standards, and the 11 remaining systems (73%) did not meet the effluent standard for onsite secondary treated wastewater. Household wastewater contains a mixture of chemicals, organic matter and other impurities. The organic matter composed of carbon consists of the digested and undigested food as well as microorganisms (Lee, Jones & Turco 2006). For aerobic micro-organisms to break down the organic matter they need to consume oxygen. Biochemical Oxygen Demand (BOD5) is a measure of the amount of oxygen that micro-organisms use as they consume organic compounds in a litre of wastewater over a five day period (Lee, Jones & Turco 2006). Lee et al. (2006) concluded that the higher the amount of BOD5 in sewage effluent, the greater the risk of the onsite system failing because high BOD5 is an indication of a high amount of organic matter in the effluent. After the wastewater is treated in the AWTS, the effluent is disposed of below ground, via a trench system (in Ballarat) where it is further treated by the bacteria in the soil. Problems occur when excess organic loading rates (above the approved standard of 20mg/L BOD and 30mg/L SS) reach the trench system, disturbing the bacterial equilibrium at the biomat-soil interface in the trench (Figure 4). The biomat is also composed of suspended solids and the by-product of microbial activity (e.g. polysaccharide residues) (Beal, Gardner & Menzies 1997). If the biomat thickens, it reduces the infiltration rate of effluent reaching the soil and subsequently the effluent can backflow to the source or exfiltrate on to the surface (Lee, Jones & Turco 2006). The cross-sectional view of a gravel soil absorption field trench (Figure 4), similar to those installed in Ballarat, shows the location of a thick layer of microorganisms, commonly called a biomat, which forms at the soil-trench interface to feed on the organic matter that flows out of the onsite wastewater treatment plant (Lee, Jones & Turco 2006). The overall results of the survey samples for BOD5 and SS for the 15 AWTSs currently operating in Ballarat reveal that 73 % did not meet EPA Victoria effluent standards. This highlights an increased risk to the environment and public health due to possible trench system failure. The life span for a trench system is between 15-20 years after which, a replacement disposal area will be required, as the soil will have reached its capacity to treat and absorb the effluent (Campelltown City Council 2007). Therefore, any treated effluent that exceeds the BOD5 and SS standard, will greatly reduce the life span of the receiving trench system. This highlights the imperative for a protected designated alternative effluent disposal area, especially for properties with area constraints. The State Environment Protection Policy (Waters of Victoria) (2003b) states that onsite sewage effluent must be able to be contained within property boundaries. In addition, the EPA Victoria Septic Tanks Code of Practice (2003a) stipulates that the disposal area (trench system) must be protected from soil compaction. This would increase the land burden from these failing systems. Failing onsite wastewater Figure 3: Overall System Performance 11,73% 4,27% Pass Fail Figure 4: Trench cross section showing biomat biomat WATER FEATURE Aerated Wastewater Treatment Systems: (continued)
Annual Review and Yearbook 2008
EHA Yearbook 2010