3. Wastewater management systems

A general introduction

• Humans live within an ecosystem – failure to manage human-generated waste within the boundaries of any ecosystem can lead to that system's failure, and such a failure will bring major costs and risks to the community.
• Human activity is part of an ecosystem – not only must wastewater management solutions protect ecosystem processes and services, but where possible they need to fit the social, economic and cultural needs of that community. Of course, protection of public health and the ecosystem must be the primary concern, otherwise the local economy and community will ultimately suffer.

Section 3 builds on these themes by looking first at what wastewater management systems might look like if they were organised within ecosystem capacities and processes. In other words, it takes a systems approach to wastewater management. The way systems might be managed over time to accommodate the changing local economic and social environment is also discussed. This time-focused or dynamic view of wastewater systems management is not often discussed, but it is an important issue for any community. This is especially true for smaller communities, which may be strongly affected by economic and social change.

3.1 A systems approach to wastewater management

Part Three will discuss the technical wastewater management systems in detail. This section briefly reviews the concept of integrated wastewater systems management that underpins this handbook.

When choosing how to manage wastewater and wastes in your community, a distinction needs to be made between the 'system' and the technical engineering solutions that might be used within that system. Thus you can talk about wastewater systems and specific wastewater treatment and disposal technologies, such as a septic tank. These are different things.

For any community, the most important thing is to realise that choosing the 'system' is the first and most important step. Choosing the technology comes second. Small communities have a wider range of wastewater systems available to them than large cities. For example, on-site systems can be used more easily. Your decision will depend on understanding the possible effects of different systems on the community's vision of where it wants to be in the future. It will also depend on the physical characteristics of the local soils and water tables; closeness to streams, rivers, lakes and the coast; and how the overall ecosystem works.

A brief history of wastewater systems in New Zealand

A wastewater management system is a human-designed and -created system to manage wastes. In New Zealand over the last hundred years or so these systems were thought of, built and managed as if they were largely separate from the surrounding natural ecosystem.

On-site systems consisted of the pit toilet and early septic tank technologies. Understandably the focus was on health issues, and overflows into waterways were not a marked concern. Often on site-systems were seen as unsafe by their very nature, waiting to be changed to a 'proper' waste management system when the community had sufficient resources. Small New Zealand communities were characterised in these early years by on-site systems and would continue with them well into the 1960s and 1970s. Controls on expansion and development because of the capacity of on-site systems also became more common. Some communities invested in outfalls for untreated wastes, often ignoring Māori protests against pollution of wāhi tapu and kaimoana. From the 1970s central government subsidies gave some funding for wastewater systems, and a large number of communities took the opportunity to invest.

The subsidies improved conditions but also brought new costs for some areas – in the form of development pressures, or extra rates for what were small communities. The subsidies were withdrawn during the 1980s, and then reintroduced in 2003 for communities of up to 10,000 population.

With the rapid increase in the size of towns and increased understanding of wastes as a source of human disease, the concern has been to transport the wastes away from towns and settlements and pass them into rivers, streams or the coast, where it was expected that dilution of the waste would occur. One effect was to concentrate the wastes and place pressure on the receiving ecosystem.

Reticulated systems that transported wastewater away from the settlement allowed some sectors of a community to forget or ignore the environmental effects, and to see management of wastes as independent of natural systems. There was no requirement to think about water quality. Issues such as soil types and water tables were irrelevant, because the system bypassed the natural process of waste management. It seemed a wastewater management system could operate separately from natural systems.

But the rivers and coastal areas were eventually overwhelmed by the volumes of waste they were expected to handle. Given that the rivers and streams were also often a source of water, from the early twentieth century there was increasing concern for the effects of contamination on human health. Developments in water supply systems, such as the use of sand-filter systems for drinking-water, allowed polluted rivers to be used as a water source.

From the 1950s, concern about effects on the ecosystem, and on amenity and recreation, forced the treatment of wastewater. In the early years this was mainly a health focus, but in later years it expanded to include treatment to a level that tried to minimise some adverse impacts on 'receiving waters'. The wastewater system still bypassed natural land-based percolation of wastes into soils, but it had been partly reconnected to the natural system by a minimum requirement to think about effects. The treatment 'bit' was added to the wastewater system.

Current wastewater systems

In the last 30 years or so the focus has been on treating wastes before they re-enter the natural nutrient and water cycle. But in recent years there has been an emerging view that rather than just 'manage the end effect', human wastewater systems should be integrated into natural processes. They have to exist within the natural system.

Figure 3.1 The evolving nature of wastewater systems

 

 

Textual description

Figure 3.2 The conventional wastewater system

 

 

Textual description

Of course, the so-called new ecosystem-focused or integrated wastewater management approach is not new. It has been used for centuries and forms the basis of Māori waste management thinking. Many smaller communities and some farms and businesses use 'on-site' systems that more closely fit this kind of approach. The non-systems approach to waste engineering of the last century was to simply focus on end-of-pipe treatment followed by disposal of the treated wastewater. Engineering and technology was then applied to meet minimum regulatory standards. Now we know that this is often an inefficient use of resources and human effort, particularly in the longer term.

Wastewater 'infrastructure' is often designed so that industrial wastewater (with all its toxic components) is mixed with our personal domestic wastewater. All this is generally piped to a single point within a community's ecosystem. The system has been designed so that all this wastewater – with its mix of valuable nutrients, toxic chemicals and pathogenic organisms – goes to one point for return to the natural system. Stormwater doesn't normally enter the same wastewater system, but is directed to the nearest waterway, not via the wastewater treatment plant. It shouldn't be a surprise that there is an ecological problem.

This approach reflects the sheer size and speed of urban development and the need to deal with the wastes that result. It occurred at a time when decision-makers thought that humans could act without thinking about the capacity of natural systems to absorb effects.

Linear approaches to problems, in which resources are used and converted into wastes, only to be disposed of, represent a failure in human ingenuity and a flaw in technology design.

Dr Steven A. Esrey, UNICEF

The New Zealand Waste Strategy (2002) takes significant steps to change the way in which wastes are regarded. A major focus is on creating a circular process which involves re-use, rather than a linear process from use to disposal.

Māori objections about direct disposal to water raised the question of the way wastewater re-entered the natural system or ecosystem. This concern has joined with a wider concern about the effects of large volumes of wastewater re-entering the environment at single points. The result is an adding of a re-entry management 'bit' to the collection and treatment parts of the system.

Figure 3.3 A sustainable wastewater system

 

Textual description

This figure outlines a sustainable wastewater system. Rainwater and water from a catchment area are stored in a lake for use by residential and commercial users. Roof water is also used by residential users. Grey water from baths, showers and other domestic use is recycled and dispersed into the ecosystem. Stormwater from commercial users is piped to silt ponds for evaporation and for re-use. Wastewater from homes and commercial users is treated in a small-scale plant; solids are composted and treated water disposed of into silt ponds. When the water makes it way from the silt ponds to the sea, it is clean.

Note: this example uses a small-scale treatment plant, but it is quite possible to manage larger systems in a more sustainable way.

In recent years, especially with limited subsidies for community wastewater systems, the costs of wastewater systems have sent some communities looking for ways to reduce the burden. This has resulted in thinking about the front end – the management of waste at source – and the reclamation of treated wastewater to provide a re-useable water source.

Finally, changing environmental standards and community expectations are demanding that communities think about the whole catchment. This is the whole valley with all the streams that flow down through it to a main river and into the sea. This means thinking about all the wastewater pressures on your local environment and trying to manage them as a whole.

In terms of designing the physical technical solutions for the wastewater system, there is now a shift from the conventional, linear, end-of-pipe technology to integrated water and waste water systems. If you want to read more about this, the report by the Parliamentary Commissioner for the Environment, Beyond Ageing Pipes (2001), gives an idea of the new thinking that is based on sustainable development principles.

Rather than linear thinking about systems and technical solutions, your community will need to think more about ecosystems and the 'water web'. A systems approach is to evaluate the whole system in relation to the social, cultural, economic and ecological environment within which it exists. Wastewater management is not only about the provision of a wastewater service. It is also linked to water supply and stormwater services.

Wastewater management is also about the wise use of our natural resources such as water, nutrients and even energy. The ecosystem, social and resource problems in urban settings are all interrelated. In particular, better management of the water resource, stormwater and wastewater can often be achieved by addressing the total urban water system.

When a reticulated system is proposed, or already exists, problems can be made worse when stormwater has not been adequately excluded from the system. Stormwater is often polluted, and adding and conveying stormwater in the same system as wastewater just adds to the volume of water the treatment system has to deal with, which creates unnecessary costs. Finally, flow patterns in stormwater are affected by rain events, which are quite different and require different handling regimes from wastewater flows.

Using seawater as the transport system for the solids in sewage is not recommended either. While this can be an attractive alternative in areas with water shortages, the salts present in seawater play havoc with the biological treatment processes and make beneficial use of the resulting sludges very difficult.

3.2 Kinds of wastewater systems

Domestic wastewater system

This is a wastewater system that processes wastewater from a home, or group of homes. The system includes the source of wastewater in the home, technologies for treating the wastewater, and technologies and processes for returning the processed wastewater to the ecosystem. Figure 3.4 is a simplified illustration of this total wastewater system for a single home. It comprises:

• the home itself – how it is built may affect how wastewater is created
• the technologies in the home, such as washing machines and toilets
• 'inputs' – such as food (nutrients), household cleaners and water
• the people and their behaviour
• the resulting wastewater
• recycling and treatment – on-site or off-site
• the ecosystem within which the home is embedded.

Figure 3.4 Domestic wastewater system

 

Textual description

Industrial wastewater system

This is a system that processes wastewater from an industrial unit, such as a factory. As with the home system, the boundaries extend from the wastewater source (the industrial processes) through to the technologies and processes for returning the processed wastewater to the ecosystem. Figure 3.5 is a simplified illustration of this. It differs from the home system in terms of:

• the types of technologies producing the waste
• the way wastewater is managed at the source
• what goes into the processes (eg, chemicals/metals)
• the kind of waste produced.

The system is similar in that it includes people, recycling, the treatment technologies and the ecosystem within which the industry sits.

Figure 3.5 Industrial wastewater system

 

Textual description

3.3 The parts of a wastewater system: fitting into natural systems

Whatever the kind of system – industrial, domestic or combined – there are four stages or parts to any wastewater management system:

• managing wastewater at source (including water conservation and recycling)
• collection and treatment
• re-use of treated wastewater and sludge
• re-entry of treated waste into the ecosystem.

Instead of the traditional end-of-pipe approach, a systems approach involves considering the total physical wastewater system, from the source to eventual return of the wastewater to the environment. This can offer more economic and sustainable solutions. For example, it may be more appropriate and more sustainable to reduce the amount of wastewater at the source by looking at the types of technologies (eg, washing machines, toilet systems) used. Or it may be cheaper and better in the long term to change the types of household cleaners used in the home to ones that do not damage your septic tank, rather than pay for complex and expensive treatment.

The scale of wastewater treatment systems

Cutting across these stages is the issue of the scale of the physical technical solutions – whether the solutions deal with one house, a business, a farm, a group of sites, or a whole community. There are three general categories of overall technical framework:

• individual: serves separate households, farms or businesses
• cluster: designed so that treatment of wastes serves groups of households or businesses, but not a whole community
• central: usually designed so the treatment of wastes for an entire community is managed at once in one place.

Variation is possible within these. For example, a cluster framework can have some on-site pre-treatment and the final treatment plant can be located off-site, or on-site among the houses. By 'on-site' is meant treatment or re-entry on the site where the wastewater was originally generated; 'off-site' means treatment away from where the waste was treated.

Re-entry of wastes can occur on-site or off-site. A centralised overall technical system can have some on-site pre-treatment, while individually focused technical solutions will always be on-site. Cluster and centralised systems tend to be managed by an overall central agency, such as a local authority. In contrast, technical systems focused on individual households or business can be managed by their users, or they can be managed and maintained as a group by a central agency.

These management arrangements, as well as more detail on the different systems, are discussed in Part Three.

3.4 National policy about wastewater treatment

The Ministry for the Environment and Local Government New Zealand published The New Zealand Waste Strategy in March 2002. It identifies wastewater management as an important issue, and signals that it wants to reduce the amount of sludge going to landfill. It has set targets for the improvement of wastewater treatment systems. You will need to read this strategy and keep up to date with the various parts of the action plan that will unfold over the next few years. It may affect your long-term decisions.

The Ministry of Health has re-introduced subsidies for the development of sewerage systems for small communities. It does not fund on-site systems. Information about this can be found in Section 11. You need to be aware of this funding opportunity, but you still need to weigh up what is the best physical system for your community.