View all publications

9. Ecosystem re-entry or re-use

Having collected and treated your wastewater (see Sections 7 and 8) you need to look at systems and technologies for its re-entry to the ecosystem. In some cases, water and biosolids can be reclaimed for re-use, and the options for this are briefly discussed below.

9.1 Re-entry of treated waste into the ecosystem

Sixty to seventy years ago the way wastewater re-entered the environment was not a major focus for communities or technicians. For on-site systems the main concern was to ensure that septic tank fields were able to absorb the wastes: periodically the tank would need to be cleaned out and the wastes buried. Various levels of treated waste from centralised systems would be discharged directly into streams, rivers or the sea. Untreated waste, especially sewage, would often be discharged via 'sewer outfalls' onto coastal areas. This, of course, has changed and significant levels of treatment now occur.

Treated wastewater may be returned to the ecosystem through direct point discharge to a water body such as a river, lake, wetland or estuary, or to sea. In this case the RMA will require high discharge standards, and Māori values often prohibit direct discharge to natural waters. Alternatively, the treated wastewater may be returned to land by various irrigation methods, such as flood irrigation, overhead sprinklers or sub-surface drippers.

Towns and cities close to the coastline tend to return the treated wastewater to the coastal ecosystem. Inland treatment plants may discharge their treated wastewater to a lake, a river, or to land via irrigation. The other waste product from a treatment plant is the processed sludge (biosolids). This may be disposed to a landfill site, spread on to land, composted, pelletised or treated for use as a soil conditioner.

Options for returning the treated wastewater to the ecosystem within the site boundaries (often referred to as on-site disposal) depend very much on the site's characteristics, such as soil types, area and slope of land available, location of groundwater, and local climate. Options include seepage into the soil sub-surface, irrigation (surface or sub-surface) and evapo-transpiration.

Types of wastewater residuals

There are four kinds of wastewater residuals that must re-enter the natural environment after treatment.

Gases

These include gases such as ammonia, methane and hydrogen sulphide, and odorous organic gases such as mercaptans, indole and skatole. These can re-enter at various points, such as if water turns septic from an overload of organic material, or at the point sludge is landfilled. Methane can build up within a site and will need to be managed to reduce risks to surrounding properties. Risk management and site management plans for landfills to manage combustible gases and odour will be an important part of the re-entry process. Often communities do not factor in the costs of landfill management into wastewater management costs when choosing options.

Wastewater aerosols

These are very small airborne droplets that can carry pathogens and other contaminants. Aerosols are created by mixers and aerators, which disturb the surface of wastewater tanks and ponds, or by overhead sprinklers. The distance these aerosols can carry in winds and the survival time of pathogens is variable and will depend on the site. A risk management plan and regulation of where and how any treatment plant or land irrigation area is to be located will be important.

Liquids

The characteristics of treated wastewater to be returned to the environment will depend on the level of treatment it has received (see Section 7).

Solids – sludge and biosolids

These can be classified as semi-solids and semi-liquids depending on the amount of water left in them. Unprocessed solids from primary and secondary treatment processes are referred to as sludges. Local authorities invest significant effort into converting sludges to biosolids and reducing the level of water in the processed solids in order to improve handling problems when they are disposed to landfills. The New Zealand Waste Strategy calls, by 2007, for such wastes to be beneficially used or appropriately treated to minimise the production of methane and leachate.

The Ministry for the Environment is placing strong emphasis on improving landfill management, and many smaller landfills have closed. Some landfills will not take biosolids. The Ministry is keen to promote re-use of biosolids, but there are issues with some processes in terms of available markets. The re-use of biosolids that have been composted is not straightforward because of concerns about the impacts of remaining heavy metals and other substances. Reference should be made to the Biosolids Guidelines. 16

9.2 Types of re-entry system

There are six main ways in which liquid and solid wastewater residuals re-enter the ecosystem, as shown in Table 9.1.

As we saw in Part One, ecosystems are dynamic, complex interacting webs of human, biological and physical processes. People are dependent on natural ecosystems for the goods, services and products they provide. Consequently our long-term wellbeing is totally dependent on maintaining healthy ecosystems well into the future. The impact of wastewater re-entry on these systems will not just depend on the quantity and quality of residuals released into them. It will also depend on the sensitivity of the ecosystems and the relative importance of the ecosystem's goods and services.

There are procedures for assessing impacts and managing them. These include assessment of environmental effects (AEE) and hazard identification analysis and monitoring programme (HIAMP). The RMA requires that these impacts be assessed before consents will be issued. The main agency for managing these effects is your regional council. These and other groups with a role in managing impacts are discussed in Section 4.

9.3 Solids re-entry technologies

On-site systems

Septage is the pump-out contents from septic tanks, and is a dilute and offensive mixture of sewage, scum and partly digested organic solids. The most effective means of handling this material is to transport it to a centralised community wastewater treatment plant, where it is processed in ad-mixture with the raw sludges produced from primary settlement tanks. Where the community plant is an oxidation pond system, the septage can be added to the facultative pond, but carefully so as not to overload the inlet zone of the pond with solids.

Table 9.1 Types of re-entry system

System Residuals managed
Freshwater ecosystems (streams, lakes and wetlands)
  • treated wastewater effluent (various levels of treatment)
Marine ecosystems (estuaries, harbours and ocean – coastal and offshore)
  • treated wastewater effluent (various levels of treatment)
Land ecosystems (agricultural, horticultural, forestry or landscaped areas)
  • some untreated wastewater (more rare)
Atmosphere
  • treated wastewater effluent (various levels of treatment)
  • odour
  • gases (indirect and flaring of landfill gases)
  • wastewater aerosols (a by-product of treatment processes)
Landfills (closed systems)
  • sludges and biosolids
Waste-to-energy plants (not used in New Zealand at present)
  • dried sludge/biosolids

Other options for septage include burial in trenches on land set aside as a management area. As the septage degrades under the bacterial action within the soil, stable humus solids are formed, and the older trenches can then be re-excavated for handling fresh septage.

Pump-out contractors are licensed by local authorities to undertake this work, and they use special vacuum-suction tanker vehicles for the purpose. A recent innovation in pump-out tanker design is a unit that dewaters the septage on-site and returns the liquid to the septic tank, while consolidating and storing the scum and sludge solids. This enables efficient long-haul servicing of remoter rural–residential areas.

Cluster and centralised treatment plants

Small community treatment plants using biofilter or activated sludge systems produce a range of sludges from the combination of both primary and secondary treatment processes. The degree of stabilisation of these solids by the anaerobic and aerobic processes in the treatment plant determines the volume of final biosolids to be managed by disposal or utilisation onto land. The wet biosolids may be dried on special sand beds at the treatment plant before being collected as dried 'cake' for trucking to land (or even to a solid waste landfill). Alternatively, the wet biosolids may be spread on land under the 1992 guidelines prepared by the Ministry of Health.

A new set of national guidelines is currently in preparation (2002) under the oversight of the NZ Water & Wastes Association. Agricultural land uses are favoured if the biosolids are digested and are mature (ie, have been aged since digestion), and can be placed by sub-surface injection into the soil. Forest land application provides an opportunity for the nutrients in the solids to enhance tree growth, and is a further beneficial use of biosolids.

9.4 Wastewater effluent re-entry technologies

On-site systems

On-site wastewater re-entry technologies are summarised in Table 9.2 below. For all of these systems the dimensions required are determined by the wastewater quantity and quality, and site conditions. Examples are given in Table 9.3. Such systems must be designed and approved by a qualified and experienced person.

The system dimensions set out in Table 9.3 represent only its design size. The site area taken up by the installed system has to include the space between each trench or mound or irrigation line, and a buffer zone around the system footprint. In addition, a reserve area should be set aside nearby for extensions to the system if needed to handle unexpected poor performance due to system overload or misuse.

Table 9.2 Ecosystem re-entry options for on-site wastewater systems

Type of system Comments
Sub-surface seepage trenches and beds This requires sub-soils with appropriate drainage characteristics. Groundwater levels must not be too high.
LPED (low-pressure effluent distribution) trenches Specially designed shallow and narrow trench systems with a nested perforated dosing pipe within a drain-coil line. Used for either deep sandy soils to distribute septic tank effluent for further in-soil treatment, or for deep topsoil conditions overlying clay to distribute effluent for topsoil treatment and evapo-transpiration.
Wisconsin mounds Sand-filled treatment mounds producing treated effluent for seepage to natural ground under the base. Used on sites with high groundwater levels and/or poorly drained soils.
ETS (evapo-transpiration seepage) beds Appropriate where soils have impeded drainage, and used in climates with good evapo-transpiration rates and lower rainfalls. Beds and/or surrounding spaces between beds are planted with high-transpiration shrubs, plants and/or grasses.
Surface spray irrigation For wastewater that has received secondary treatment (AWTS, packed-bed reactors [an upgraded intermittent sand filter], wetland) plus disinfection via ultraviolet light or chlorine tablets. Note: Not used for on-site applications in NZ.
Surface drip-line irrigation For wastewater that has received secondary treatment (AWTS, packed bed reactors [an upgraded intermittent sand filter], wetland). Drip lines are laid on the soil surface and covered with mulch or bark or compost. Can be designed for incorporation within a landscaped area on the lot.
Subsurface drip-line irrigation For wastewater that has received secondary treatment (AWTS, packed bed reactors [an upgraded intermittent sand filter], wetland). Drip lines are laid within good topsoil to depths of 50 to 100 mm. Can be designed for incorporation within a landscaped area on the lot.

Table 9.3 Indicative dimensions of on-site ecosystem re-entry options

Sand loam – free draining Loam – moderate drainage Light clay – poorly drained
  Water saving Trenchlength(m) Mound area(m2) Sub-irrigation(m2) Trenchlength(m) Mound area(m2) Sub-irrigation(m2) Trenchlength(m) Mound area(m2) Sub-irrigation(m2)
Primary effluent No 120 40 NA 174 45 NA 400 112 NA
Yes 72 23 NA 106 28 NA 240 70 NA
Secondary effluent No 40 20 180 50 23 225 200 90 315
Yes 24 11 180 31 14 138 122 55 190

Source: Based on AS/NZS 1547:2000

Centralised and cluster technologies

Water re-entry

Discharge of wastewater effluent to natural water such as streams, rivers, estuaries, harbours and the ocean has traditionally been used by most of New Zealand's larger communities that have developed alongside or in close proximity to such waters. Early drainage systems were based on the 'dilute and disperse' approach to using natural self-purification processes in the water ecosystem to treat the wastes. As communities expanded, treatment plants were provided to reduce the polluting impact on the receiving waters. In addition, rather than use end-of-pipe discharge into natural waters, special diffusers were used to achieve better dispersion and dilution of the treated effluent. For example, ocean outfalls for the larger coastal urban areas have been substantially upgraded by improved treatment and diffuser systems in recent years. The main forms of community wastewater effluent re-entry used in New Zealand are shown in Table 9.4

However, cultural issues associated with Māori spiritual values, together with the recognition that water re-entry systems often do not provide sound environmental performance, have shifted the emphasis for new or upgraded facilities away from water re-entry towards land re-entry. This shift in approach has been particularly significant for smaller communities, as the land areas needed can be more readily found in the adjacent rural areas than can be found for a larger community. For large communities upgrading their treatment and ecosystem re-entry systems, the use of constructed or natural wetlands has been accepted as an appropriate buffer between the treatment plant and the natural water into which the final discharge diffuses.

Land re-entry

Land options include rapid infiltration, overland flow, and low-rate irrigation by either spray irrigation or drip-line irrigation. Land treatment is the favoured method for achieving the cultural objectives for human waste management by the majority of Māori iwi.

Table 9.4 Main forms of wastewater effluent re-entry in New Zealand

Form of re-entry Number of communities %
Freshwater
  • stream flow
147 51.9
  • lake
4 1.4
  151 53.4
Marine
  • estuarine
7 2.5
  • harbour
13 4.6
  • coast
6 2.1
  • offshore outfall
29 10.2
  55 19.4
Land and other
  • to land
59 20.8
  • land/ excess flow to water
17 6.0
  • pipeline to another treatment plant
1 0.4
  77 27.2
Totals 283 100

Rapid infiltration can be both treatment and 'disposal' (via discharge to groundwater some distance below the soil infiltration surface). Partially or fully treated effluent is soaked into the ground at a high rate for further in-soil treatment. Only sandy soils are suitable for long-term use, and the water table must be sufficiently deep so that all human bacteria are trapped in the soil, where they can gradually die off and not contaminate the ground water. You should note that other pathogens may not be removed.

Low-rate irrigation is a land-treatment and disposal system that involves total effluent absorption via soakage and evapo-transpiration through planted crop or vegetation ground cover. Application rates are only a few centimetres per week, so large land areas are required. The higher the level of pre-treatment (secondary treatment being a minimum), the more effective the long-term performance of the irrigated area in coping with the effluent load. For spray irrigation systems, significant buffer distances (planted, non-irrigated borders) are required adjacent to any location where people may be present to avoid human contact with aerosol-carried bacteria in the spray drift.

Forest irrigation is a common method of effluent spray irrigation management, with the advantage that nutrients and water enhance tree growth. Grassland spray irrigation is another method, but unfortunately the dairy industry is not interested in using the harvested crop for fodder as they say that overseas consumers are likely to reject dairy product from cows fed on human effluent-irrigated pasture. Where drip-line systems are used, buffer distances can be very small, and horticultural use of the treated effluent nutrients and water becomes feasible.

In-land treatment via surface application and under-drainage lines for collecting filtrate that is subsequently disposed to a receiving water or to a reclaimed water use is a variation on rapid infiltration. It can provide the advantages of irrigation for crop or pasture growth where water table depths may restrict application rates unless lowered by artificial drainage.

9.5 Re-use of water and biosolids reclaimed from wastewater

Traditionally wastewater has been managed as a product that is a threat to both human and ecosystem health. Consequently, the infrastructure design for handling such a material will reflect this.

Domestic wastewater contains essential resources such as water, nutrients and organic material. Treated wastewater produces liquid wastewater and primary and secondary sludge, which is the material that remains once the original water-borne waste is 'dewatered'. Both these wastes can be processed to recover reusable water and composted biosolids for horticultural application as a soil conditioner.

Figure 9.1 Alternative reuse strategies

This figure shows alternative re-use strategies after treatment for biosolids (gas for energy via anaerobic digestion; energy extraction using heat pumps; compost material) and reclaimed water (irrigation; wetland restoration; non-potable purposes).

Re-use of biosolids requires a higher level of treatment beyond what is achieved with the normal treatment of primary and secondary sludges.

A number of technologies are commonly used that utilise the resource value of wastewater, most commonly with centralised systems, where the volumes of treated wastes are likely to be large enough to encourage investment. It is also possible with the smaller cluster systems, although this is a fairly new area. Re-uses include biogas production for energy (a process that converts the organic component of primary and secondary sludges to methane), irrigation of water and wastewater nutrients for biomass production, and the use of the treated wastewater for wetland restoration. Other practices overseas include aquaculture, energy extraction (from the wastewater) by heat pumps, urine separation, and nutrient stripping for the production of nutrients.

It is rare for an on-site system to involve re-use, although some of the options include recycling treated wastewater or greywater for non-potable uses such as toilet flushing and irrigation, or feeding landscaped wetlands, and the use of composting toilets and production of humus.

Reclaimed water has non-potable uses for garden irrigation or industrial processes. Wetland restoration involves artificially putting water back into a wetland to offset the loss of water from drainage of surrounding areas and the lowering of the water table.

Re-use of reclaimed water is a new part of wastewater management in New Zealand. It is also where Māori have concerns about the re-entry of wastes. There are concerns about irrigation direct on to food crops, and uncertainty about compost as an end use. Non-potable use is acceptable if it is not used for food production, and where it must pass through soils first. There is also wider community concern about some of these processes (eg, heavy metals in composts).

Health authorities also have concerns regarding the use of reclaimed water sourced from wastewater because of the possibility of direct contact with pathogens if something goes wrong with the treatment process, or if the system is not adequately maintained.

A wide range of technologies can be explored, even if the area is relatively new. Like managing water use at source, biosolids and reclaimed water re-use have the potential to reduce the overall cost of the wastewater system. For a smaller community it may be worth looking at how the waste streams, especially sludges to be converted to biosolids, might be combined with other communities in a centralised process. Re-use is well worth exploring as part of your wastewater thinking.

Golden Valley subdivision, Kuaotunu, Coromandel Peninsula

A new subdivision of 40 residential lots has been designed and constructed (2000) with a pumped MEDS (modified effluent drainage servicing) collection system. Filtered septic tank effluent is conveyed in 50 mm pressure sewer lines from a pump within each septic tank to a central recirculating sand-filter treatment plant located in an enlarged and landscaped central median strip on the access road serving the development. The very high-quality effluent produced is in part disinfected and returned to each lot as non-potable reclaimed water for toilet flushing. The remaining effluent flow is not disinfected, but pumped to an area of steep terrain where it is to be irrigated by driplines into eucalyptus planted plots. A portion of treated effluent will be held in storage for firefighting purposes.

The advantage of the recirculating sand media filter treatment system for this type of development is that it can be commissioned to run on a modular basis. Treatment capacity can be extended to match housing numbers as constructed over time. On a seasonal basis, modules can be started up and then shut down to fit the expansion and contraction of holiday occupancy. All this can be accommodated while maintaining a consistently high treatment performance.

Because of the use of a fully sealed reticulation system, there will be no infiltration into the system, thus protecting the treatment plant from excess flows. The treatment plant performance, including the operational status of all mechanical units and effluent quality readings from treatment stages, is remote monitored by sensors, with the resulting information transferred to computer surveillance at the operating company's headquarters in Auckland. This is a design-build-operate (DBO) project where the performance of the overall treatment system is remote monitored by offsite specialists, but with locally trained service people on standby callout to deal with any operational events that need attention.

Footnotes:

16Guidelines for the Safe Application of Biosolids to Land in New Zealand Copyright © New Zealand Water Environment Research Foundation 2003. return