Black water comes from your toilet. A septic tank is the most basic way of treating black water ( primary treated) with solids settling into the bottom of the tank and floating grease and scum to the top of the tank. The waste water flows out of the system from the ‘clear zone’ between these two layers directly to the land application system (primary treatment). On GBI, because of the lack of reticulated power, we have a special dispensation which allows us to discharge waste water which is primary treated only. Everywhere else in the Auckland region waste water must be treated to a secondary or tertiary treated standard. Secondary treatment involves a range of different mechanisms which agitate and oxygenate the waste water thus allowing “good” bacteria to breakdown the “bad” bacteria. These systems use power, some can be very complicated, and some have a very poor long term success rate. If you wish to discharge your waste water through pressure compensating drippers you would need secondary treatment as a minimum.
Two examples of secondary treatment used on GBI would be sand filters, where the waste water is filtered through a sand medium before being distributed through drippers, and Textile filters where the waste water is sprayed on textile filters before being distributed through drippers. These provide a secondary treatment by providing good conditions for aerated bacteria to further treat the waste water. They are also a possible solution for more sensitive areas. The disadvantage of secondary treatment systems is that they can use a lot of power and some are quite complicated requiring regular maintenance.
Tertiary treatment systems go a step further than the secondary treatment systems and can involve the addition of chlorination and UV treatment.
The type of application system used is again site dependent. Land application systems must be pressure dosed. Gravity can be used in some circumstances or a pump may be used as the situation requires but the main concern is that whatever method is used the discharge is evenly disposed over the entire disposal area. The Auckland Council recommends pressure compensating drip irrigation lines (PCDI) where treated waste water is pumped in doses into the PCDI system which consists of a number of dripper lines just under the surface of the ground. Unfortunately because of power considerations this may not be the best option for some people. Most land application systems use the KISS principal,” Keep It Shallow Stupid”. Shallow trenches can be used as a method of land application and evaporation with the waste water being distributed near the surface where there is the most oxygen and organic matter.
Due to the hilly nature of the Island and the power restrictions, some households use a low pressure effluent distribution system (LPED) where a series of shallow trenches are made across a hillside with adequate top soil and then dose loaded either by pump or siphon effect. These systems are restricted to 15 degree slope maximum. This works well but requires adequate topsoil, regular maintenance, and can be interfered with by tree roots.
As with all black and grey water solutions, each site will favour one or other solution from TP58 (page 64). TP58 is the Council guidance document on on-site waste water systems which includes information about various treatment options including alternatives to septic tanks such as composting, grey water and vermiculture systems.
Septic system example cost
Septic system installations require consent. Ask your plumber or contact the Auckland Council.
Basic septic system.
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Last updated on the 06-04-2020