Low Cost Sewerage Schemes

There are many small coastal communities in Queensland which currently use individual septic systems and discharge primary treated effluent to the groundwater. In many cases the effects on the environment are measurable. The lack of adequate sewerage treatment is often a barrier to further development of the real estate in these communities. The cost of a standard sewer reticulation system and a standard centralised treatment system is often prohibitive. There are affordable options for both reticulation and treatment of effluent, that have been widely used elsewhere. These include Septic Tank Effluent Treatment Systems (STETS). These system may be gravity or pumped or a combination of both. A STETS system in combination with an alternative waste water treatment plant can produce a cost effective option which will greatly improve the environmental outcome and allow development to proceed within the community.

A prefeasibility study is required when considering the option of an alternative sewer arrangement. The study should include the following aspects:

  • A desk top assessment of the topographic, geological, geographic and existing land uses and tenures relevant to the investigation area.
  • A site inspection.
  • Make recommendations based on the above and report.
  • Detail a conceptual design following the above to a level that a budget costing can be prepared.
  • Summarise the Statutory Regulatory Process going forward.
  • Identify potential partnerships and  funding opportunities. Discuss the cost allocation of various parts of the proposed scheme.
  • Consider possible head works charges and development charges that may apply.
  • Summarise potential risks and their mitigation. Risks will include; financial, environmental, health, engineering, amenity and approvals.

A great deal of information can be obtained by visiting a site and carrying our some basic soil testing using hand held equipment. Where possible road cuttings and existing trenching should be observed. A preliminary assessment of any difficulties related to the installation of deep sewers can be made. Catchments for the sewer reticulation scheme and potential sites for pump stations, rising mains, a centralised sewerage treatment works, possible decentralised treatment sites, land disposal sites and creek/river discharges can be identified and the position fixed using GPS. Permeability testing utilising a permeameter can be undertaken at potential sites for land disposal. The information can be identified using GPS and combined with existing databases.

Rainfall and evaporation data is readily available and will assist in determining the potential for various land disposal options.

Attitudes to STEP and STEG collection systems can be assessed and recorded. The potential for effluent re-use will also be considered.

Site contours can be obtained from available sources such as Landgate and a potential reticulation arrangements can be developed and the route walked. An extensive photographic record should be compiled. Any design work at this stage will be in sketch format only.

The issue of population growth will need to be investigated and the likelihood of implementation of the proposed sewer system generating additional growth will be considered.

The likely hydraulic loadings to the proposed sewer system will be investigated. Reticulated water demand can be used as a guide to this. Typically 140 litre/person/day allowance for the collection systems and 180 litre/person/day for the treatment system applies based on South Australian experience. These figures are escalated for peak flow and infiltration with the factors depending on the system components and local climate and soils type.

Any local flood information will be obtained. Where the information is informal then this will be compiled as a spatial record using a GPS.

The value and opportunity for effluent re-use will be assessed. Where re-use is a possibility then the level of sophistication warranted will be evaluated. Re-use can range from watering of local parks and gardens to agricultural irrigation and aquifer re-charge. The effects of runoff in the wet season and the need for seasonal storage will be considered. It is unlikely that effluent re-use will apply in the wet tropics.

Following the site visit the information collected will be collated and assessed. Some preliminary sketch designs will be compiled and indicative costing’s prepared for several options. If warranted a cost matrix will be compiled for the different possible combinations.

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