Whether the overall wastewater system you choose is a centralised system, cluster system, on-site system or some combination of these, there are some things people can do at the source that can be adopted to ease or reduce the cost of the ultimate treatment and ecosystem re-entry system requirements. While this section will only address domestic wastewater issues, there are also source management options for industrial, trade and commercial wastewater systems. Contact your local council for information to assist you in this area.
Options for management at the source include:
- water-saving practices in and around the home
- choice of household products that will enter the wastewater stream.
The amount of water used by a community will be a major factor in deciding the size of a wastewater system. Fairly obviously, water conservation can reduce the amount of wastewater that needs to be dealt with. It is possible to calculate whether water conservation will affect the system design and final costs.
The design of a wastewater system must also take account of what materials are going down the drains. The presence of different toxic materials may demand a higher level of treatment than would normally occur. What goes down the drain also has a huge impact on how well septic tanks and on-site systems work. Again, a full-system review, which manages the amount of toxic materials, greases, fats, oils, etc. going down the drain, will influence the design of the final system.
Some kinds of wastewater can be re-used even before they leave the house or business to be treated. To do this it is worth thinking about the four kinds of water that are part of a household system:
- water: for drinking, washing, cooking (potable); for transporting wastes (non-potable); and for other uses such as watering gardens and washing cars (non-potable)
- greywater: from baths, washing machines, showers and sinks
- blackwater: human wastes (urine, faeces and blood)
You can reduce the amount of water used for potable and non-potable purposes. This reduces the amount of greywater and blackwater being created, and therefore the amount needing treatment. It is possible to re-use greywater and stormwater for non-potable purposes. Stormwater may enter the pipes on your section that are carrying the wastewater for treatment, and this can also be managed.
Reclaimed water sourced from greywater and stormwater cannot be used for cooking, washing or drinking.
An important issue will be the ability to change or 'retro-fit' older houses and businesses. It is worth your community looking at how much wastewater is being produced and how much this can be reduced before estimating how big your treatment processes need to be. You need to consider:
- How much water is being used, and can you reduce it?
- What are the opportunities to re-use?
- How much stormwater is getting into your system?
Your local council can probably help you with the information needed to find answers to these questions.
Alternatives for urban water and wastewater management, North Shore City
In recognition of the holistic approach taken to wastewater management by its Project CARE 13 working party, North Shore City commissioned a study into alternative technologies for household water conservation, excreta disposal, and stormwater management. The project evaluated a wide range of technical, environmental, social (including public acceptability) and financial criteria, from which a short list of the most promising technologies was formulated. The outcomes of the study were reported in March 1999.
The servicing scenarios considered included partial use of the community sewerage system by uncoupling blackwater for on-site treatment and disposal (with greywater to the sewer), cluster systems (on-site primary treatment, off-site secondary treatment for groups of dwellings), and full on-site management. However, current provisions under the Building Act do not enable disconnection from an existing sewer service.
Four shortlisted schemes were costed. Schemes A and B are based on 'greenfields' development. Some of the technologies used could be retrofitted into properties within existing urban areas as well as being applied to new infill housing. Schemes C1 and C2 could be applied to existing (and infill) urban areas. However, the Ministry of Health does not recommend the use of composting toilets in urban areas.
Scheme A: Individual roof-water supply, greywater recycling, individual on-site wastewater disposal, communal stormwater capture and storage for firefighting (cluster of 15 properties):
On-site water and wastewater facilities $19,500 (per property)
Communal stormwater treatment and storage $3,333 (per property)
Total per property cost (1999) $22,833
Scheme B: At-source separation of urine (with communal collection and recovery), communal capture, treatment and storage of roof water, greywater and communal stormwater and recycle for toilet flushing and firefighting storage; blackwater on-site septic tanks plus EDS transfer of effluent for communal cluster treatment and drip irrigation (cluster of 15 properties) [Note: Cost excludes the 4,000m2 land area required for effluent management]
Total per property cost (1999) $20,800
Scheme C1: Waterless composting toilet retrofit (per property) $3,000
Scheme C2: Composting and greywater vermiculture system (per property) $13,000
Minimising the quantity of wastewater
Water use within a property boundary will either be for internal or external purposes. Water used for external activities – such as irrigation, car washing or swimming pools – does not normally enter the wastewater stream. Figure 6.1 shows the water use for a single family unit, as presented in a Christchurch City Council Water Conservation Report. This report suggests that, on average, internal use accounts for 60% of the annual water use and the remaining 40% is external. However, the proportion of water used for external purposes will vary considerably. A home with a swimming pool, or boats and cars to wash or gardens to irrigate, will use very large quantities of water compared to a home without such demands. (See Appendix 3 for more information on water use.)
Reducing water use within the home through modifying individuals' behaviour will not only lead to reduced water consumption, but also to reduced wastewater production. Clearly not all water-saving measures will reduce wastewater volume (eg, wise lawn and garden irrigation, mulching to conserve water, fixing leaking outdoor taps). Water-saving actions that will reduce domestic wastewater volumes include fixing indoor dripping taps, reducing showering times, and avoiding wasteful teeth-cleaning practice. The major internal water consumers are the toilet, laundry and shower (see Figure 6.1). Showers generally do save water compared to baths.
Two pie diagrams show internal (60%) and external (40%) use. Internal breaks down to leakage (5%), faucets (19%), laundry (20%), toilet (30%), dishwasher (2%), shower (21%), baths (3%). External breaks down to leakage (1%), cleaning (11%), pools/fountains (2%), car washing (5%), and irrigation (81%).
Minimising the polluting content of wastewater
In addition to wastewater volumes, the polluting qualities of the wastewater are important when considering the measures that might be taken at source to reduce pressure on the wastewater management system, and the final impact when the treated wastewater is returned to the ecosystem. For example, a garbage grinder can increase the biochemical oxygen demand (BOD, a measure of the polluting strength of wastewater) by over 35% (see Section 2.3 for information on BOD).
The products that home-owners flush into their wastewater system include paints, pharmaceutical mixes, antibiotics, hormones, oils and volatiles, pesticides, herbicides, detergents, cleaning agents, polishing agents and other products with active ingredients which can be both a health risk to humans and impact detrimentally on wastewater treatment and eventually ecosystems.
Dr Robert Patterson, Director of Lanfax Labs, Armidale, in Australia, has been researching the effect of different household products on wastewater management for some years. He points out that: 14
The range of household products available for disposal to the sewer is uncontrolled. While stringent requirements are placed on liquid trade waste discharges, domestic discharges are immune from either monitoring or control.
Patterson also notes that the two major effects of using laundry detergents are elevated sodium salt levels in wastewater and a significant increase in alkalinity. Both effects will result in destruction of soil structure if applied to land. Swedish research suggests it is possible to reduce the phosphate in our wastewater by more than 40% if everyone uses low-phosphorous household products.
The elements contained in cleaning agents are detailed in Table 6.1.
Examples of other ingredients from household products that can enter the wastewater stream are listed in Table 6.2. This is by no means a comprehensive list. In addition to these, heavy metals such as zinc from sun-screens, copper from copper pipes, mercury from dentistry, and chromium from metal plating can enter the wastewater stream and will accumulate in the sludges produced by the treatment plant.
Table 6.1 Cleaning agent elements
- washing powders, toilet cleaners and dishwasher powders usually contain chlorine-based bleach. The chlorine can combine with organic compounds to form highly toxic, and carcinogenic, organochlorine compounds. Non-chlorine bleaches are available, and washing powders with separate bleach are a good choice (for several reasons)
- phosphates are added to washing powders (but not to washing-up liquid) to soften the water. They can lead to eutrophication in watercourses
3. Optical brighteners:
- these substances do nothing for cleanliness but give an illusion of whiteness. Problems include allergic reactions, poor biodegradability, and mutation and inhibition of bacteria in your treatment system
4. Other additives:
- NTA 15, EDTA, enzymes, preservatives, colourings, synthetic fragrances, etc. are all suspect in terms of ecological impact in production and final disposal
- possibly a more benign replacement for phosphate, it is an inert mineral, but it can encourage algae problems in seawater
- common salt (sodium chloride) is used as a thickener in washing-up liquid, and soaps and detergents contain sodium ions, which break down the structure of clay soils and so reduce their permeability. This can be a problem for leach fields and greywater irrigation
Source: Reference: N Grant, M Moodie, C Weedon. Sewage Solutions. Centre for Alternative Technology, Machynnlleth, Powyrs, 1996, p. 125.
While Patterson urges that re-use initiatives start in the supermarket, it is also an option for communities to support education and awareness programmes to encourage good household practices that will result in a healthier and more sustainable wastewater cycle, which will be more integrated with the local ecosystem.
|Alkyl benzene sulfonates (ABS)||Common surfactant in laundry detergents, cleaners.||Very slow to biodegrade; the manufacturing process can release carcinogens and toxins to environment.|
|Alkyl phenoxy polyethoxy ethanols (also nonyl phenols)||Used as surfactant in laundry detergents, cleaners.||Slow to biodegrade in the environment; linked with chronic health problems.|
|Butyl cellosolve (also, butyl oxitol, ethylene glycol monbutyl, butoxyethanol, ethylene glycol)||Used as solvent in spray cleaners, all-purpose cleaners.||A toxic synthetic – can irritate mucous membranes and cause liver and kidney damage.|
|Chlorine – also as hypochlorite, sodium hypochlorite, sodium dichloroisoctanurate, hydrogen chloride and hydrochloric acid||Household bleaching agent.||Most frequently involved in household and industrial poisonings. Reacts with organics in the environment to form carcinogenic toxins, the most well known being dioxin. Serious impact on small wastewater treatment plants.|
|EDTA: ethylene-diamino-tetra-acetate||A builder used as phosphate substitute in detergents.||Slow to biodegrade.|
|Formaldehyde||Not a common ingredient these days but may be found in deodorisers, disinfectant, germicides, chemical toilet additives, particle board.||Extremely potent; carcinogenic and respiratory irritant; serious impact on small wastewater treatment plants.|
|Methanol||Used as solvent in glass cleaners.||Acutely toxic and can cause blindness.|
|Phosphates||Used in detergents and cleaners as a builder and deflocculating agent.||Non-toxic but major cause of eutrophication in receiving aquatic ecosystem, causing serious ecological imbalance.|
|Polycarboxylates||Laundry and dishwasher detergents as an anti-redeposition agent.||Not much known; non-biodegradable and petroleum based.|
This is a growing area for home-plumbing systems and the design of appliances. Some of these are listed in the tables in the next column.
The advantages and disadvantages of various toilet designs are given in Table 6.4.
|Reduce the amount of water used in toilets - reduces amount of black water|| |
|Reduce the amount of water that becomes grey water|| |
|Recycle and re-use water before it becomes wastewater|| |
|Conventional flush||6–15||Single flush||Low cost; high water use; good range of systems available|
|Dual flush||0.5–6||Two flush options||Low cost; medium water use; good range of systems available.|
|Vacuum toilets (discharge to vacuum sewer)||0.5–1.5||Separate vacuum unit required||Low water use; expensive; would have to import systems into NZ; limited range (can only be used in conjunction with a vacuum sewerage collection system)|
|Urine-separating (discharge to urine-holding tank)||0.2–4||Separate plumbing for urine and for faeces||Enable recovery of nutrients; not common in NZ; requires separate urine-handling system|
|Hybrid or micro-flush (toilet pedestal located on top of pre-treatment tank)||< 0.3||Very small quantity of water used to flush||Very low water use; available only from Australia; separate greywater system required|
|Composting||0–0.1||No water used||Not flushed after use; cleaning instructions are maunfacturer specific; requires on-site management of compost and separate greywater system|
|Dehydrating||0||No water used||No water used; requires on-site management of removed solids and separate greywater system|
|Incineration||0||No water used||No water used; requires on-site management of removed ash and separate greywater system|
Changing to water-saving appliances or components can also result in significant reduction in wastewater output (Table 6.5 gives examples).
|Appliance||Volume of water used|
In 1997 the NZ Consumer (No. 356) stated that compared to top-loading washing machines, front-loading machines used less water, energy and detergent.
For more information on water-saving technology and management, see Appendix 3.
There is increasing interest in composting toilets in some sectors of our community. Available commercial composting toilets range from simple to sophisticated designs. From an environmental viewpoint there are clear advantages with these systems: the water use is substantially reduced, and nutrients and organic matter can by recovered to re-enter the natural nutrient cycle. However, a well-designed and easy-to-maintain composting toilet may be more expensive than conventional flushing toilet systems. Composting toilets also generally require regular management, and a greywater servicing system will be required in addition to the composting toilet.
Composting toilet experience in New Zealand has led the Ministry of Health to conclude they are not appropriate for full-time household use on residential-sized lots. The most successful systems are those in holiday recreational areas where controlled management can be provided. The Ministry also points out that once reticulated sewerage is provided, then composting toilets cannot be used under the Building Act.
See Appendix 4 for more information on composting toilets.
13 Project CARE is North City’s $250 million, 20-year project aimed at establishing and meeting the community’s beach water quality expectations. return
14 RA Patterson. Reuse Initiatives start in the Supermarket. In Proceedings, NSW Country Convention, Institution of Engineers, Australia. 6-8 August. Northern Group, Institution of Engineers, Australia, Armidale. return
15 Note: NTA (nitrilotriacetic acid) and EDTA (ethylenediamine tetraacetic acid) are both chelating agents that help to control water hardness ions that can interfere with the performance of household, industrial, and institutional cleaning products. return