On-site sewage system danger

Installing on-site sewage systems in the wrong soil types can lead to potential environmental and health hazards, a Queensland University of Technology researcher has warned.

Les Dawes, from QUT's Faculty of Built Environment and Engineering, has found that many of the 20,000 on-site sewage systems located on the fringes of Brisbane, Logan and the Gold Coast performed poorly as a result of soil conditions.

"Due to inappropriate siting and inadequate evaluation of soil characteristics, the failure of these systems has become a common scenario," he said.

"In order to achieve sustainable on-site wastewater treatment with minimal impacts on the environment and public health, there needs to be a more appropriate means of assessing the long-term performance of on-site dispersal areas."

Mr Dawes said on-site sewage systems were used in areas where homes were not connected to a centralised wastewater collection system.

Mr Dawes said on-site septic systems works by settling the sewage and then allowing the effluent (treated wastewater) to drain into the soil which acts as a filter and removes harmful bacteria and nutrients.

"Soil can be a wonderful cleaning agent," he said.

However, Mr Dawes warned that not all soil types were suited to adequately remove the unsafe bacteria and nutrients.

"Sandy soils are the worst and the Gold Coast is built mostly on sand which is a concern" he said.

"In some of these built up areas the on-site systems are on small 600 square metre lots of land and if the effluent is not being properly treated by the soil, it has the potential to cause health problems."

Mr Dawes said when high levels of nutrients reached surface water such as streams and creeks it could lead to outbreaks of algae.

"The best soil types to site on-site systems are the well structured moderately permeable clay soils yet many areas of south-east Queensland have very sandy or heavy clay soils."

Mr Dawes said the only way to fix the thousands of existing systems was to retrofit septic tanks with effluent filters before the effluent reaches the soil. This is currently done on new installations.

"But the problem with this is monetary and a lack of resources at council level," he said.

"Instead what councils need to be doing is selecting the right areas to allow on-site sewage systems to be sited.

"If we get it right first and place on-site systems on soil that is capable of filtering wastewater then we won't have this problem."

Suburbs where these systems are located include Jacobs Well, Cabbage Tree Point and Coomera on the Gold Coast, Mt Cotton, Forestdale, Park Ridge and Greenbank in Logan and Brookfield, Moggill, Aspley and Bellbowrie in Brisbane, to name just a few.