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12. Criteria for selecting a wastewater servicing option

This handbook aims to provide sufficient information to enable communities and individuals to participate in making decisions about the best wastewater servicing option for their community. The options vary from a larger centralised wastewater servicing system to individual on-site wastewater systems. There are a variety of options for both the individual technological components of such systems and for the ways these different individual technologies can be fitted together to provide a total wastewater servicing system. There are also several options for the way these systems might be managed. All these factors can influence the decision on which is the best system to install.

This handbook emphasises that a wastewater servicing system can be linked to ecosystem services such as water supply, stormwater, and food and fibre production (via the nutrient cycles), as well as social and cultural services such as education and research. These factors are discussed in Section 1.

These various interrelated issues can make the process of selecting the best option very complex. To enable more holistic decision-making, and a better integration of services within these human and natural ecosystems, we have provided a framework for decision-making in this section. The basis of this framework is illustrated in Figure 12.1. From this is derived a series of criteria for evaluating the various options. The information required for these criteria has been provided in earlier sections of this handbook.

Scoping the options

Two levels of option assessment are offered to help you with your decision. Each site will have certain characteristics that will eliminate particular options.

The first level of evaluation, given in Tables 12.1 and 12.2, is an initial scoping exercise to eliminate the options that are clearly not suitable. The second level of evaluation, given in Table 12.3, provides more detailed criteria against which a reduced number of options can be assessed.

The detail of the criteria for evaluating the different options for wastewater technologies and wastewater system is extensive, complex and site-specific. As a result, it is strongly recommended that as a community you:

  • identify your own goals in relation to your need for wastewater systems
  • set your own criteria for evaluating the different wastewater system options
  • identify indicators that would enable ongoing monitoring of the chosen system.

Figure 12.1 Criteria for selecting options

This figure summarises the criteria for selecting options. A system approach is about selecting the option that best fits the total natural and human ecosystem within which it is embedded. The human system includes individual developments, governing systems and social/cultural systems – all of which are interlinked and all link to the funding and economic system and infrastructure. Questions to be considered include: (from individual development) Does it achieve integration with the water web and 3-waters? How does it impact on individual members of the community? Is it resilient to natural hazards? What are the total and ongoing costs and how will it be funded? (from governing system) What are the impacts on the ecosystem services? What are the management requirements and are they appropriate? (from social/cultural system) Does it protect public health? Does it meet social and cultural criteria? Does it meet existing and changing expectations, needs and minimise risk? How well does the option manage environmental impact residuals?

Table 12.1 Scoping the options: conventional systems – benefits and limitations

Type of System Brief description and possible benefits System and site limitations
On-site: basic treatment system Wastewater is treated and then discharged within the property boundaries. Treatment is usually by a simple septic tank system followed by some dispersal system such as sub-surface trenches or mound.

This is the lowest-cost on-site option. If well designed, can be reliable with minimal maintenance and operational requirements

These systems are likely to be inappropriate on properties with the following limiting factors:

  • very small section
  • steep sloping section
  • high ground-water table at any time during the year
  • very poorly draining soils or rocky section.

It can be an expensive option in urban areas, with costs of $4,000–$7,000 per standard home, and $50 to $100 annual maintenance costs

On-site: high quality treatment Wastewater is treated and then discharged within the property boundaries. Treatment may be by an active aerated system or multi-chamber septic tank, followed by a sand-filter system.

The treated wastewater is of higher quality and can be dispersed by sub-surface irrigation, and is therefore better suited to sites with poorly draining soils. Irrigation effects can be beneficial

These systems are likely to be inappropriate on properties with the following limiting factors:

  • very small section
  • steep sloping section
  • poor surface drainage.

It may cost $8,000–$13,000 per standard home and up to $150/year operating and annual costs.

Cluster The wastewater from a collection of local houses, or other activities, is reticulated to a nearby treatment plant where it is treated and then returned to land, usually within the site area set aside for treatment and ecosystem re-entry.

Cost sharing can mean lower cost per connection while maintaining a high quality of treatment. Water recycling is made easier; loading to centralised system is reduced.
This is best suited for a housing development specifically designed for a cluster system. It requires a local area of suitable land for the treatment plant and re-entry of the treated effluent to the ecosystem.

Adequate soil types, groundwater conditions and topography are required. An appropriate management and servicing structure is required. Costs are variable, and will depend on design, site and number of connections.
Centralised All wastewater is collected at the source and then transported (through sewer pipes) to a central site for treatment and final return to the ecosystem. This may be the lowest-cost option (although full environmental costs are often not factored in). Management and control are very easily centralised. This is not appropriate in sparsely populated areas (eg, rural areas) due to cost. Because such systems can involve very large wastewater volumes, there may be site limitations in providing a sustainable ecosystem re-entry technique. Costs per property are usually less than on-site options.

Table 12.2 Scoping the options: less common systems – benefits and limitations

Type of system Brief description and possible benefits System and site limitations
Reclaimed water recycling Reclaimed water sourced from treated wastewater effluent can be recycled for non-potable water uses, although this requires a high standard of treatment. Such systems include multi-stage treatment, recirculating sand filters and disinfection. Recycling of reclaimed water for on-site toilet flushing, laundry and car washing and irrigation is possible. This is an appropriate option to consider if potable water is expensive or in short supply. Careful consideration must be given to potential health risks. Such systems require a high standard of treatment, disinfection and management. Separate and clearly labelled plumbing and outlets are necessary.

No NZ guidelines are yet in place to cover such on-site recycling uses, so local authorities are unlikely to grant approval until the Ministry of Health has assessed risks and devised appropriate guidelines for risk elimination.
Composting toilets Composting toilets are waterless (dry) or minimal water use (wet) toilets that use aerobic bacteria and other micro-organisms to biodegrade the faeces and other organics. There are various designs suitable for outdoor installations (eg, forest parks) and domestic installations. Modern composting toilets are designed for domestic use as clean, odourless facilities. Benefits include low water use, and recycling of organic matter and nutrients Well-designed domestic composting toilets can be expensive and require competent, consistent and dedicated management. Some require sufficient under-floor clearance for the composting chamber. The composted solids require handling and appropriate safe burial. Most composting toilets will not accept greywater, so a separate and approved greywater system will be required. Many councils are reticent about approval because of perceived health risks if compost toilet systems are not properly operated and maintained. Compost removal must be undertaken to strict hygiene standards, so regulatory obstacles often face people seeking to use this type of system.
Vacuum toilets Vacuum toilets for domestic applications are not common in NZ (the only system is at Turoa Skifield, Mt Ruapehu). They have been used in countries where water is expensive or in short supply. These toilets use very low water volumes (0.5–1.5 L/flush). Benefits include low water use and wastewater volumes. Concentrated blackwater offers better technological opportunities for nutrient recovery (eg, liquid composting). The vacuum unit, toilets and vacuum pipes are expensive and require skilled installation and design. For some people the noise of the vacuum can be off-putting, although recent designs have eliminated this problem.

Technology and expertise are not common in NZ.
Separated systems: greywater blackwater, faeces and urine Separation of the various wastewater components enables separate management and recovery of the water and the wastewater nutrients. Most nutrients are contained in the urine, while most of the water is in the greywater. Urine-separating toilets are available and plumbing can be installed to separate these streams Separate plumbing is required and will increase building costs. No cost benefits are gained if connected to centralised system. On-site systems require suitable treatment systems for each component, and land area and soils for ecosystem re-entry. Many councils are not familiar with these options

Table 12.3 Examples of detailed criteria for assessment

Physical characteristics of the site:
  • limitation of site or area (eg, soils)
  • resilience to natural hazards.
Ecological:
  • effect on habitat
  • effect on ecosystem services
  • effect on waterways
  • effect on marine ecosystems
  • effect on overall natural systems
  • ecological restoration opportunities
  • resource efficiency – closing of ecological cycles.
Compatibility with Māori perspectives:
  • issue of passage onto land
  • protection of mauri.
Other cultural concerns:
  • sensitivity to other cultures
  • local stewardship/responsibility
  • potable re-use of treated water
  • inter-generational issues.
Public health:
  • operational safety
  • effects of failure on community health
  • residue and human proximity.
The technical system:
  • reliability
  • serviceability
  • engineering life of the system
  • resilience to acts of vandalism
  • linkages with other opportunities and services (eg, water supply).
Ability to be changed:
  • extendability
  • flexibility
  • adaptability.
Management:
  • convenience
  • operation and maintenance implications.
Economic factors:
  • capital costs
  • ongoing annual costs.
Community effects:
  • level of local control
  • need for external expertise/management.
Community change:
  • pressure for future growth
  • capacity to absorb growth
  • declining population
  • ageing population
  • visual and noise effects.
Other potential benefits:
  • leisure and recreation
  • education
  • research.
Formal processes:
  • familiarity to decision-makers
  • technical demands
  • differing demands
  • ease of the consent process.

An example of a matrix showing some of the above criteria evaluated against the broad wastewater services categories is given in Appendix 7.