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10. System configuration, performance and failure issues

In Sections 6 to 9 different parts of the wastewater management system were described, from managing wastewater at source, through collection and treatment, to its re-entry into the ecosystem or re-use for various purposes. In this section we will be looking at options for how some of the overall systems come together, and how they perform.

Many smaller communities will have on-site systems, and your decision-making will centre around how these work, can they work better, and what your options are for moving to cluster or centralised systems. For this reason the section has a strong focus on on-site system configurations, their advantages and disadvantages. However, we also compare the performance of cluster systems and on-site systems, and conclude by running through the typical reasons for on-site system failure and how to avoid it.

Table 10.1 On-site wastewater system options

System Source control Wastewater Treatment Disinfection Loading Ecosystem re-entry
1 No water saving All waste (black and grey) Conventional septic tank (ST) NA Trickle loading Conventional seepage trench
2 No water saving All waste (black and grey) Multi-chamber ST (MST) or improved single-chamber large ST with effluent outlet filter (IST) NA Dose loading Conventional seepage trench or LPED trench system
3 No water saving All waste (black and grey) MST or IST NA Dose loading Wisconsin mound
4 Water saving All waste (black and grey) MST or IST NA Dose loaded Seepage trench or LPED trench system
5 No water saving All waste (black and grey) MST or IST plus intermittent (ISF) or recirculating sand filter (RSF) NA Dose loading Sub-surface irrigation
6 No water saving All waste (black and grey) MST or IST plus constructed wetland None Dose loading Sub-surface irrigation
7 No water saving All waste (black and grey) AWTS None Dose loading Sub-surface irrigation
8 No water saving All waste (black and grey) AWTS UV or chlorine tablet Dose loading Surface irrigation
9 No water saving All waste (black and grey) MST or IST Storage of compost for 12 months Spray loading

Dose loading
Evapo-transpiration seepage (ETS) bed
10 Composting toilet and water saving Blackwater Composting None Manual Returned to topsoil as humus
Greywater Grease trap and constructed wetland   Dose loading Subsurface irrigation

10.1 On-site wastewater system configurations

The on-site technologies described so far in Part Three can be combined in a variety of different ways to provide an on-site wastewater servicing system. Table 10.1 presents some of the combinations that have been used.

Each of these is described in more detail in a series of key features diagrams in Appendix 6. The detail in Appendix 6 is provided so that you can see how each system works, and to help you make informed judgements about the kind of system that best suits your community. (Note: combinations other than the 10 listed in Table 10.1 may be possible.)

10.2 System performance

Table 10.2 provides a summary of some of the effluent qualities provided by various on-site and cluster treatment plants. Regional council rules will often set the discharge quality requirement for a range of treatment technologies relative to the council's oversight of environmental effects from discharges. Councils have responsibility for managing the potential cumulative effects of wastewater servicing (either on-site, cluster or centralised) on the natural land and water environment. On-site systems come under the permitted activity rules of councils, but all cluster and centralised treatment plant discharges will need to be processed via council consents procedures, and issued with a discharge permit to which conditions will be attached (including the effluent quality to be met).

Table 10.2 Performance of different treatment technologies17

On-site systems Cluster
  Raw domestic wastewater Septic tank AWTS Sand filter SBR Extended aeration Constructed wetlands Packed bed sand or textile filter
BOD5g/m3 200-300 120-150 15-40 5-15 3-9 < 30 5-15 < 5
Suspended solids g/m3 260-400 40-120 20-60 5-20 2-19 < 30 5-20 < 5
Total nitrogen g/m3 30-80 40-6018 25-50   2-9 < 7 5-30  
Total Kjeldahl Nitrogen (TKN) g/m3 30-80 40-60 25-50 30-50     5-30  
Total phosphorus g/m3 10-20 10-15 7-12 5-10 1-10 < 8 5-10  
Faecal coliform cfu/100ml 106-108 103-105 10-103 10-103   < 104 300-1000 1000
Note:

Some of the systems shown are able to treat raw effluent directly (septic tank, AWTS, SBR, extended aeration); others are secondary and/or tertiary systems requiring some sort of preceding treatment (sand filter, constructed wetlands, packed beds).

Footnotes:

17 Many of these systems can also be designed in different ways and built with different sizes to achieve different treatment objectives (eg, a large, constructed wetland will generally work better than a small one treating the same flow).

18 T Gardner, P Geary, I Gordon. Ecological sustainability and on-site effluent treatment systems. Australian Journal of Environmental Management 1997, 4: 144-156.

10.3 On-site system failure

Table 10.3 provides a selection of potential 'failure modes' (things that can go wrong) for on-site wastewater servicing. Failure in this context is defined as the inability of the system to perform as intended by the design. Either poor soil assessment during the design phase, incorrect design, inadequate attention to installation, or lack of operation and maintenance servicing can initiate such failure. Improper use by overloading the system with more people than it was designed for, or the discharge of substances such as fats or paints or chemicals down the inlet sewer line, will all contribute to failure.

Table 10.3 Failure in on-site systems

Treatment failure
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.