2. Wastewater – What is it?

2.1 The kinds of waste

With a focus on wastewater you will be most interested in liquid wastes, but sludges, odours and other residuals are also very important. Liquid wastes include:

• wastes that originate as natural liquids, such as urine or drinking-water, or from laundry uses in a house
• a mixture of wastes and water used for their transport – wastes are mixed with water as the 'medium' by which they are transported away from a site for safe disposal. In a sense, a large part of wastewater is 'created' as a transport system.

The relationship between solid and liquid wastes is shown in Figure 2.1.

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Understanding these relationships is important to understanding the available choices. You don't have to use water to transport wastes (eg, sewage or human waste can be managed with composting toilets). The costs of providing water to transport sewage, or the need to dispose of a large volume of wastewater, may lead some communities to explore alternatives to water-borne waste treatment. This kind of thinking is fairly new in New Zealand and it will not always be the cheapest, most useable option. But understanding that there are choices about 'creating' wastewater is important and worth considering when exploring options.

Wastewater includes dissolved contaminants, suspended solids and micro-organisms. Various levels of wastewater treatment separate the wastewater into sludge and a dissolved fraction containing much of the water, organic material, bacteria and salts. What is left behind is called sludge or biosolids. It is important to see sludge as one of the wastewater management issues for your community. Often sludge is ignored as a wastewater problem because community attention is solely focused on treating the remaining water to a level that reduces harm to waterways, and the nutrient cycle in particular.

Liquid waste, especially wastewater containing human wastes, will also produce an odour (from gases and aerosols). Odour is not a public health issue, but it can be a major source of nuisance and concern in a community. It will be part of your wastewater management challenge.

As will be obvious by now, the three types of waste (solid, liquid and gas) overlap in a variety of ways. The focus in this handbook is on the management of wastewater (liquid waste) and sludge. These overlaps, and the focus, are illustrated in Figure 2.2.

Figure 2.2 The overlaps between the different types of waste

 

 

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2.2 What is in wastewater?

It is important to remember that wastewater is what goes down the pipe, and that the management of wastewater includes its impacts and infrastructural requirements. Your community therefore needs to think of itself as managing:

• the total impact of all wastewater in your surrounding catchment on public health and natural systems and processes
• the physical systems (infrastructure) that channel some of these wastes for treatment and controlled 're-entry' back into the ecosystem.

If you take the total impact aspect first, there are probably four broad sources of wastewater in New Zealand, each with its own mix of substances that eventually find their way into wastewater:

• household systems
• factories and industry
• commercial businesses/offices
• farms and horticulture.

Wastewater from farming tends to be dealt with separately. Increasingly, farmers are being required to set up on-site treatment systems for such things as animal effluent. Households, industries and commercial businesses can all use on-site systems, but often, depending on the size of the community, their wastewater will be combined and managed together.

This means that the wastewater in your area will be unique. An essential factor in determining the kinds of wastewater you will need to deal with will be the kinds of industries and processing businesses in your area. For example, if there is a local cheese factory, your wastewater system will have to deal with whey as a waste. If there is a metal-processing factory, your system may have to deal with water that has been used to wash down machinery.

Wastes from industry and businesses are known as tradewastes. It will be important to take account of these tradewastes when designing your system, and important to take account of initiatives being undertaken by industry to reduce the volume and toxicity of their wastes. It would be worth working with these industries in order to help them to deal with their own waste streams.

You will need to understand the mix of what is in your local wastewater and what effects the various components may have on human health and ecosystems. We will turn to look at this now.

2.3 What is in wastewater that causes problems?

Organic material

The organic content of wastewater is made up of human faeces, protein, fat, vegetable and sugar material from food preparation, and soaps from cleaning. Some of this is dissolved into the water and some exists as separate particles.

Ecosystem health effects

Naturally occurring soil and water bacteria eat this organic waste and use it to grow rapidly. In a natural or dilute water environment where there is plenty of oxygen dissolved in the water, aerobic (oxygen-using) bacteria eat the organic material and form a slime of new bacterial cells and dissolved salt-waste products. If undiluted wastewater is left on its own, however, anaerobic (non-oxygen-using) bacteria decompose the waste organic material and release odorous gases such as hydrogen sulphide, as well as 'non-smelly' gases such as methane and carbon dioxide.

It is the amount of oxygen removed or the too-rapid growth of the bacterial slime that can cause the harm (see below).

The important thing is to measure how much oxygen will be used by aerobic bacteria to convert the organic material to new bacteria. This is the 'biochemical oxygen demand' (BOD), and the standard measure is the amount of dissolved oxygen needed by aerobic bacteria over a five-day period at a water temperature of 20° Celsius (called the BOD₅). The BOD₅ strength of wastewater indicates its potential polluting impact if it is not treated. It is measured in parts per million (ppm), or in the metric system the number of grams of organic material per cubic metre (g/m³). The BOD₅ of untreated wastewater is around 200–300g/m³, while the BOD₅ for a healthy aquatic ecosystem would be less than 5g/m³.

Relating these scientific measurements to everyday experience, a central issue is how much oxygen is left for fish to breathe after aerobic bacteria have used the oxygen to break down the organic material. If BOD₅ levels of less than 4 g/m³ occur in a stream that has naturally healthy levels of dissolved oxygen, then the stream system can deal with the amount of waste without affecting the fish. A good-quality healthy level of dissolved oxygen in water is around 8 to 10 g/m³. At a dissolved oxygen level of 5 g/m³ the fish become stressed, and at 2 g/m³ the fish will die from lack of oxygen unless they are able to move to more oxygenated waters.

Figure 2.3 The effects of organic material and nutrients released into waterways

 

 

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Where there is an overwhelming amount of wastewater, all the oxygen will be used up and the anaerobic bacteria will take over. The water will go septic (anaerobic) and the fish will die, as will other forms of oxygen-dependent life. This is partly why wastewater is treated to remove as much organic material as possible. But the content of even treated wastewater can be an issue for your community. Sensitive streams and estuaries are particularly vulnerable.

In effect, ecosystem services can be damaged, and these problems may be felt well before the level of pollution directly affects human health. For your area, you will need to know how much wastewater is entering – or may enter – your local stream or river, and the level of dissolved oxygen. Talking with the regional council may help with this.

Suspended solids

The portion of organic material that does not dissolve but remains suspended in the water is known as suspended solids. The level of suspended solids in untreated wastewater is around 200 g/m³.

Ecosystem health effects

If effluent is discharged into streams untreated, any solids it contains will tend to settle in quiet spots. Oxygen levels will soon be depleted in the area of the contamination, causing it to decompose anaerobically. If there are high concentrations of this contamination the water in the stream will go septic because the oxygen will be used up. This will not only smother the fish, but will also kill off the life at the bottom of the stream, creating dead zones.

Dissolved salts

The most significant salts in wastewater are nitrates and phosphates. These occur naturally to some extent. Nitrate also derives from the breakdown of organic nitrogen in protein waste matter, and the oxidation of the ammonia in urine. Phosphates are present in detergents used in washing and laundering, and are also produced by organic breakdown. The total nitrate in wastewater is around 40 g/m³, and phosphate is around 15 g/m³.

Ecosystem health effects

Nitrates and phosphates are essential elements for growth. When nitrates and phosphates are discharged into natural waters they fertilise the growth of microscopic algae and water 'weeds', which can lead to green algal suspensions and weed mats. This overgrowth results in their death and decay, and means further consumption of dissolved oxygen and smothering of aquatic life. The nutrients that caused the initial growth can then be released back into the water, initiating another cycle of weed and algal growth and decay.

Bacteria and viruses

The human gut produces a huge quantity of bacteria, which are excreted as part of faeces on a daily basis. The most common and easily measured organism is E.coli (Escherichia coliform group), which is referred to by wastewater scientists and engineers as 'faecal coliform' bacteria. This is called an 'indicator' because its presence indicates the presence of faecal matter from warm-blooded animals. More extensive testing is required to tell if the source is human or not.

The amount of faecal coliform is measured per 100 ml of water – around half a cup. Each person excretes about 140 billion faecal coliforms a day. In untreated wastewater the faecal coliforms can be around 10 to 100 million per 100 ml. It is the presence of these faecal coliforms that the drinking-water standards and recreation standards are concerned with.

The main class of viruses are the enteric viruses, which cause gastro-enteritis; for example, calcivirus (Norwalk virus), rotavirus, enterovirus (polio and meningitis) and hepatitis. Generally viruses do not replicate in the outside world, but they may survive for a long time. Spray irrigation may shock viruses into die-off due to exposure to ultraviolet light or drying out of their surroundings. Poliovirus 3 has been found in aerosols at a wastewater treatment plant .³ In a marine environment some viruses have been known to survive a number of days, possibly protected in suspended solids.

Human health effects

Many of the faecal coliform bacteria in human waste are harmless. However, there are disease organisms – or 'pathogens' – that can cause harm. These can be bacteria such as typhoid, or viruses such as hepatitis B. Direct contact with these pathogens or pollution of the water supply can cause infections. The Ministry of Health has national responsibility for developing drinking-water standards, which will guide your community's understanding of the risks it might face from local wastewater. Sewage can pollute shellfish-gathering areas and, if eaten, the shellfish will cause illness. Shellfish filter food by passing several litres of water an hour through their system. The food concentrates in the shellfish, which means that any pathogens will also accumulate.

Relatively high concentrations can also make an area unsafe for swimming and 'water contact recreation'. National guidelines developed by the Ministry for the Environment help local communities to classify their harbours, streams and lakes in terms of safety for swimming, fishing and shellfish gathering. Local regional councils will set standards for discharges for these areas. These standards relate to the amount of bacteria present in a certain volume of water.

Ground water can also become contaminated. Wastes can percolate through the soils into underground water or aquifers. Given that many smaller communities and farms obtain their water from bores or wells into these aquifers, this contamination can be a serious issue.

During the nineteenth century the large quantities of sewage in the bigger towns and cities were identified as a health problem. Finding solutions to cholera epidemics from infected water supplies was a major issue. The wastewater system you now have may well be a direct heritage of these concerns.

Other dissolved constituents

Wastewater contains metals, chemicals and hormones from households (via food, medicines, cosmetics and cleaning products) and business processes (eg, mercury from dentistry, which can easily be removed by installing a centrifuge in dental surgeries). It can also contain halogenated hydrocarbons and aromatics, plasticisers, polyaromatic and petroleum hydrocarbons, organochlorine pesticides, PCBs and dioxins.

There are two issues: if large quantities are discharged into small, highly localised areas, such as a stream or small lake, there may be pollution problems. The other issue is the 'bio-accumulation' of these substances in various parts of the food chain. This can bring unacceptable concentrations in humans and aquatic life, which can lead to health problems.

Human health effects

Long-term health impacts of residues in water supplies and food

The issue here is one of long-term impacts of various wastewater residues on the human system. Water naturally contains such things as iron, zinc and manganese, but industrial processes can introduce higher concentrations. If the concentrations are high enough, exposure to some metals and chemicals may have an impact on how the body's system works.

The long-term impacts of these substances on human health are not always well understood. Wastewater will carry a range of substances, which can pass into the water supply or be returned to the soil in heavy concentrations. Some treatment systems will remove metals and chemicals from the wastewater, but the sludge produced as a result of this treatment will then contain a high concentration of these substances. The New Zealand Waste Strategy calls for such wastes, by 2007, to be beneficially used or appropriately treated to minimise the production of methane and leachate. Whatever use the sludge is put to, it should comply with the Biosolids Guidelines. ⁴

Endocrine disruption

The endocrine system in the human body is a complex network of glands and hormones that regulate many of the body's functions, including growth, development and maturation, as well as the way various organs operate. The endocrine glands – including the pituitary, thyroid, adrenal, thymus, pancreas, ovaries and testes – release carefully measured amounts of hormones into the bloodstream, which act as natural chemical messengers. They travel to different parts of the body to control and adjust many life functions.

An endocrine disruptor is a synthetic chemical, which, when absorbed into the body, either mimics or blocks hormones and disrupts the body's normal functions. This disruption can happen through altering normal hormone levels, halting or stimulating the production of hormones, or changing the way hormones travel through the body. This is a new area of scientific investigation and is not yet well understood. There are concerns that, for example, the decline in fertility levels in all animals in the food chain, including humans, could be as a result of excessive discharge of these chemicals. Such investigations are now being considered in New Zealand.

The issue is relevant to wastewater issues because many of these substances will enter the food chain – either on land or in waterways – from wastewater. Of course some of the chemicals (eg, some pesticides) will also enter the ecosystem via run-off from farms and roadways. Wastewater treatment systems will remove some of these chemicals, but generally treatment processes are not currently designed to deal with this problem.

Ecosystem health effects

Endocrine disruption

The issue raised for human health is also relevant to aquatic ecosystems. There is some concern that the hormone-producing systems in fish are under pressure. High levels of oestrogen released from wastewater can affect the reproductive cycles of fish. The degree to which this is an issue in New Zealand is not known.

Toxic effects on freshwater and marine life

These can have the immediate effect of killing fish, invertebrates and even plant life. This can be a serious loss in itself, but there are also flow-on effects. The dead fish or plants will be broken down, and can contribute to further depletion of oxygen in the water.

The key point to remember is that wastewater management is not just about toilet flushing, bathing, cooking and washing water. It is likely your community will have tradewastes, even if just from the local garage. Your overall catchment will have a huge variety of different wastewaters that will need to be considered. Table 2.1 summarises the different components of wastewater that cause problems.

2.4 Other effects of wastewater

Soil depletion

This is not so much an effect of something in the wastewater itself, but has more to do with how the management of nutrients in wastewater systems bypasses natural processes. It is worth discussing here because of the link with ecosystem health.

Over the last hundred years or so waste management design has favoured using water to transport wastes. It has also favoured direct disposal into rivers, lakes and the sea. The remaining sludge has tended to be landfilled. One effect has been to bypass the nutrient cycle, whereby wastes would be slowly returned to the soils to be taken up as a food source by plants. Some would enter the streams and rivers via groundwater but most would remain in the soils.

The depletion of nutrients from the soils has been raised as an issue in parallel with a wider concern with sustainable environmental management. This depletion means that if soils are to successfully support plant life (and farming), they must have nutrients returned through alternative processes. This can be costly.

In effect, bypassing the natural nutrient cycle means that many wastewater systems contribute to nutrient depletion in soils. Conversely, streams, rivers and lakes face risks from overloading with nutrients – with many of the problems mentioned earlier.

Soil structure

Sediments, metals and salts can affect soil structure. For example, sodium ions can be found in high concentrations in wastewater. If irrigated on to land they can damage soil structure.

Summary

2.5 Māori perspectives on waste and waste impacts

Naturally Māori would have concerns about the impacts of chemicals, metals, human sewage and other effluent on the physical health of ecosystems, especially food-gathering areas, and these are likely to align with the concerns identified by the wider community. Further to these, however, is the traditional significance many Māori continue to place on the health of the mauri (see Section 1.5) within human and other life forms, including water, and the extent to which this is likely to be irreparably harmed through the introduction of unacceptable biological influences.

For example, the spiritual harm that is considered to result from consuming shellfish collected from water that may have been subject to effluent and other pollution is unacceptable to many Māori. These types of concerns are particularly prevalent in relation to the treatment and subsequent disposal of biological waste from hospitals and funeral parlours. Similarly, the use of human waste in products, including fertilisers, intended for agricultural and food production (even if these have been treated and mixed with other organic matter) can be problematic, particularly for Māori who maintain a traditional view of environmental protection.

As discussed in Section 1.5, from a traditional Māori perspective polluted water needs to pass through the earth to be purified and to have its mauri, or essence, restored. This is considered necessary, irrespective of whether treatment to remove or dilute pathogens, chemicals and metals has occurred. Even human waste found in treated wastewater must first pass through the earth before re-entering any water. In effect, it is the process undergone for treatment that is the issue, as much as the removal of pollutants.

Responding to these concerns requires us to focus on how wastewater and sludge should re-enter the ecosystem. Land re-entry appears to be the preferred approach, with wastes entering the soils before they become absorbed by plants. What this could mean for wastewater management systems is explored later in this document.