Table 10.3 Failure in on-site systems
| Failure mode | Avoiding failure |
|---|---|
| Sludge buildup on septic tanks or AWTS will reduce treatment performance | Regular checking of levels of both settled and floating sludge in the various chambers with desludging when necessary will prevent failure. |
| Filter blockage – for treatment systems with proprietary filters (septic tank filters, or filters pre-sub-surface irrigation). | Regular checking and cleaning of filters. |
| Sand filter clogging – where septic tank effluent is treated further before discharge to re-entry system | The system should be designed by a competent wastewater engineer. It is important that the sand dosing arrangement ensures regular and uniform distribution of the septic tank effluent over the surface of the sand filter. |
| Biological failure – most on-site wastewater systems rely on small micro-organisms, such as bacteria, to breakdown and purify the wastewater. If these living organisms are poisoned by chemicals flushed down the kitchen sink, toilet or other drains, the treatment system will fail. | Take care with what chemicals are flushed down drains and toilets. Do not flush disinfectants, oils, thinners, paints, bactericides, fungicides, pesticides or chlorine-based cleaners. Use biodegradable cleaners and detergents as much as possible. |
| Leakage – from or into a below-ground tank and pipe-work can cause failure. High groundwater table and stormwater infiltration can cause hydraulic overloading | Ensure the subsurface tanks, gully traps, and pipes are well sealed from surface stormwater and groundwater flow. |
| Ecosystem re-entry failure | |
| Blocked drainage field – this may occur due to one or more of the following factors; inadequate pre-treatment, poorly draining soils, or overloading of the field drainage system. | Ensure that the total system is designed by a competent wastewater engineer who carries out a thorough site investigation. It is important that systems are installed by experienced and competent trades people. |
| Flooding due to high groundwater – groundwater levels will vary throughout the year. A flooded field drainage system will cause system failure. | Design and install a system that can cope with high groundwater conditions. |
| Blockage of irrigation distributors – sprinklers and drippers. Most NZ wastewater irrigation systems use subsurface drippers. These may block after some time in operation. Emitters with small apertures can block due to the lodging of suspending particles in the treated wastewater and/or growth of bacterial slimes and other micro-organisms in the pipeline. Fine roots may also penetrate dripper apertures. | A high-standard pre-treatment
is essential for an irrigation system (AWTS or better) Suitable upstream
filters must be installed. Some dripper lines come impregnated with
biological growth inhibitors to prevent biological growth and root penetration.
Regular checking is advised and dripper line replacement recommended when blockage occurs. |
| Mechanical failure - pump failure or electrical outabe | Regular maintenance is advised
where effects on the environment and human health have been identified Back-up systems should be designed as part of the system |
| Surface ponding – a poorly designed and managed system may result in the treated wastewater ponding on the ground surface. This can be a serious health risk. | The reason for surface ponding must be determined and corrective measures taken. It can be due to a high groundwater table, overloading, blocked drains and impermeable soils. |
| Overloaded system – extra flow over and above the design allowance floods the soakage system and results in breakout of wastewater and ponding on the ground surface. This can be a serious health risk. | This could occur if water saving devices are no longer operable or used, or if an extra water using fixture such as a garbage grinder is installed when not allowed for in the design. |
