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.
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).
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.
