8. Treatment systems
Section 6 looked at managing your wastewater at source, in terms of reducing both its quantity and polluting content. However successful you are at achieving this, you will still need to collect wastewater and treat it before it can be returned to the ecosystem. Section 7 looked at the collection alternatives for conveying the wastewater to the treatment process. This section now looks at the systems and technologies for providing that treatment.
8.1 Treatment: what does it do?
Wastewater treatment processes are used to:
- remove possible contaminants from the water used to transport the wastes – the end product of this process is treated water and sludge/ biosolids
- reduce the amount of water in the remaining sludge/ biosolids so that they can either be landfilled or re-used more easily.
Different stages of wastewater treatment can be used to reduce pollutants. The options used will depend on:
- the kinds of pollutants present in the waste
- decisions about the cultural effects
- the ability of the receiving environment to absorb the waste
- the total effect – not just of your community's wastes, but of possible wastes from elsewhere.
The main point to note here is that each stage removes only certain kinds and levels of pollutants, as summarised in Table 8.1.
Sludge from primary treatment is smelly, grey-black, semi-solid – stuff! It contains high concentrations of bacteria and other micro-organisms, many of them carrying the risk of disease, as well as large amounts of biodegradable material. It will mean dissolved oxygen in water will be used up very quickly.
Secondary treatment will produce secondary sludge. This is made up of the micro-organisms that have eaten the original wastes. It is not quite as nasty as primary sludge, but does contain high levels of pathogens (disease-causing) and material that will decay and cause odour.
You will need to understand what is in your waste and what your receiving environments can 'take' or absorb as a first step in choosing your technical treatment systems. This is the heart of the natural systems approach.
8.2 Types of treatment systems
The treatment component can be located either on-site or off-site. Off-site treatment may be one large treatment plant for the whole village, town or city. This is referred to as centralised treatment. Some off-site systems only do primary treatment and they rely on discharge through long outfalls to the sea. This is becoming less common, especially as the Department of Conservation and regional councils move to improve the quality of the coastal environment. A wide range of different technologies is now available and used at centralised sewage treatment plants, ranging from simple oxidation ponds to high-tech physical, biological and chemical treatment processes.
At the other end of the scale is on-site treatment. Once again a range of treatment options is available. The conventional systems include septic tanks (these may be multi-chamber with filters), and more advanced systems such as aerated wastewater treatment systems (AWTS), recirculating sand filters, or sand-mound systems. Other, less common systems include constructed wetlands, sphagnum peat mounds, and separated grey- and blackwater systems (eg, waterless composting toilets and vacuum toilets). If waterless toilets are used, on-site greywater treatment will also be required.
Cluster treatment systems serve a small number of wastewater suppliers. For example, a common treatment plant may serve a housing development with several houses. In populated areas cluster treatment plants tend to be compact, low-maintenance, odour-free systems. If the cluster treatment plant can be located some distance from the built-up area, treatment may be by oxidation ponds and/or treatment wetlands.
There are variations to both the centralised and cluster treatment configurations. For example, it may be more economical to provide some on-site pre-treatment, such as a septic tank or grinder pump, which would allow the use of a lower-cost small-bore collection pipe network to the cluster or centralised treatment unit.
To sum up, there are four general kinds of treatment systems that deliver these different treatment processes:
- individual (on-site) treatment systems
- central treatment
- cluster treatment
- a combination of on-site and centralised treatment.
Each of these is dealt with below in turn.
Individual (on-site) treatment systems
These service individual sections or lots where all waste produced on-site is treated on-site. Generally the treated waste re-enters the ecosystem on site. This means the ability of the soils to absorb the treated waste will determine whether this kind of system can be used.
See
Figure 8.1 at its full size and textual description.The nature of your local groundwater systems, including the level of the water table in different seasons, will be important. Sometimes underground water (aquifers) can be affected by wastewater trickling though the soils and polluting the water. This water may find its way into a local stream, or bores may bring it to the surface for household use.
Some soils will not be suitable. Others may require a larger area for absorption. In many ways the absorption ability of these soils will have been a major factor in originally deciding the density of your community's settled areas. Deciding whether or not to stick with on-site systems will be a 'crunch' issue for your community. The ability of the soils to absorb wastes at all, or to absorb increased amounts, will be a deciding factor for the system you choose. This handbook will give some guidance on the general kinds of issues with soils, but expert advice will be needed.
On-site systems use biological processes that need to be carefully managed and protected. People can find this tiresome, and some visitors to beach communities may know little about how to deal with them. There are ways the community can come together to manage the separate on-site systems. In other words, individual systems do not have to mean private management. The modern approach to managing on-site systems involving system monitoring and operation and maintenance inspections (see Section 11) can ensure the long life of the system while protecting the investment in the system hardware. The cost of this managed approach can, when spread out on an annual basis, equate to the sort of charge that councils levy as sewerage charges in urban residential areas.
Treatment systems can also be designed to deal with different kinds of wastewater. For example, on-site systems can deal with a combination of greywater and blackwater, just greywater, or just blackwater. Details of on-site technologies are provided in Section 8.3.
Central treatment
This is almost the exact opposite of on-site systems. All waste is collected and transported to a central treatment site, and then re-enters the ecosystem. This kind of system can deal with household waste and tradewaste if certain concentrations of chemicals and other substances are controlled. This protects the pipes and the treatment plant and reduces the amounts that might remain in the sludge. If the sludge is to be re-used for such things as compost, then the issue of concentrations becomes very important.
Centralised systems are designed to service a complete town or settlement. The size and complexity of the system is dependent on the size of the community. They tend to involve an extensive pipe network: usually the system is fed by gravity but it often involves pumping stations as well.
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Figure 8.2 at its full size and textual description.The costs of setting up a centralised system can often be cheaper than on-site solutions because the costs are spread across many households and businesses. But the maintenance costs for the complete system can be high and often make up 15–20% of a council's annual budget. Therefore you need to think about the ongoing costs as well as the initial set-up costs.
Other considerations also need to be taken into account, such as the increased volumes of sludge produced by centralised systems, not to mention the increased need to rely on chemicals to stabilise the biological treatment that takes place. All of these issues need to be given a full advantage/disadvantage analysis that is beyond the scope of this handbook.
Cluster treatment
The focus here is on relatively small treatment plants designed to service a group of houses or businesses. More than one plant may be needed to service the whole community. They provide considerable flexibility. For example, your community may decide that it wants to continue with on-site treatment and the densities of settlement that this brings. At the same time, it may be prepared to allow a one-off development of a certain size that cannot be serviced by on-site systems. Provided the development has its own cluster system, it can proceed.
On the other hand, it may be that your community is on a centralised system. To allow more growth would require a bigger system – not just the treatment plant but the pipes as well. This can be expensive. More development might be possible if a small cluster system is used. It is therefore a useful tool for allowing some growth and change to occur without shifting to a centralised system that might bring pressure for even more growth. Often a cluster treatment system utilises land disposal. The area of land needed will be determined by the number of dwellings serviced by the cluster system. At the same time, a cluster system can allow a more managed land-based ecosystem re-entry because the volumes of waste treated will be relatively small.
Cluster treatment can also be linked to a centralised system. For example, some technologies allow the 'mining' of wastewater, by hooking up to wastewater mains pipes and removing some of the wastewater for processing. This mining can provide reclaimed water for re-use and contribute to reducing the amount of wastewater going to a centralised plant.
Combination of on-site and centralised treatment
A combination system means that:
- wastewater can be collected at source before it is piped off-site
- some basic pre-treatment occurs on-site
- many variations are available
- communities can 'mix and match'.
On-site systems such as septic tanks are sometimes seen as 'old-fashioned' systems that should be replaced where possible. Fully centralised systems are sometimes seen as something that a community should aspire to. This is changing as the possibilities of cluster systems become better known. On-site systems can seem a bother for land owners because they require a lot more direct care. Sometimes communities will choose a more centralised system to avoid these day-to-day problems – even if the local soils can still deal with on-site systems.
It is important that these four categories are not seen as inevitably moving from the primitive to the modern. Each one is equally important and capable of delivering a safe, efficient treatment. The real issue is what system best fits your particular physical environment (especially soils and water quality) and social circumstances.
See
Figure 8.3 at its full size.8.3 Types of treatment technologies
Where wastewater is collected for off-site treatment in a central or cluster wastewater treatment plant, a number of conventional treatment units are available, either single-process or a combination of primary/secondary/tertiary treatment processes. For combined on-site/off-site treatment the on-site component is usually a septic tank or improved septic tank, and the resulting effluent is conveyed to the cluster or central plant for secondary treatment.
On-site treatment technologies
Our discussion will focus on domestic on-site wastewater treatment technologies. The treatment component follows wastewater collection, and precedes the technology for returning the treated wastewater to the immediate local ecosystem (re-entry).
Treatment does not significantly reduce the volume of the incoming wastewater, so an unchanged volume has to be returned to the land within the site. It does, however, reduce or transform the dissolved and suspended constituents of the wastewater, mostly by physical processes (such as settling or filtering suspended material), or by biological degradation by bacteria of both suspended and dissolved material in the wastewater.
Treatment plant performance is usually given in terms of measures such as the total concentration of suspended solids (TSS); the five-day biochemical oxygen demand (BOD5), which provides an indication of the concentration of dissolved and suspended organic material); a particular nutrient concentration (eg, total nitrogen or total Kjeldahl nitrogen); and the concentration of faecal indicator bacteria (faecal coliform E. coli). (See Section 2.3 for further explanation.)
See
Figure 8.4 at its full size and textual description.Figure 8.4 illustrates the performance of three different on-site treatment systems: septic tanks (ST), an aerated wastewater treatment system (AWTS) and the sand filter. Their performance is expressed in terms of the five-day BOD and suspended solids (SS). These can be compared to the quality of the raw (RAW) incoming domestic influent.
The figure shows that the standard septic tank does not produce a high-quality effluent, whereas the AWTS and sand filter produce a better-quality effluent. This is why sub-surface irrigation (with drippers with small holes) of septic tank effluent will fail, because even if the drippers don't block up with the carry-over of suspended solids, they are likely to become blocked by the growth of bio-films in the dripper line due to the poor quality of the effluent.
Centralised and cluster treatment technologies
Table 8.4 summarises the various treatment processes commonly used for cluster and small centralised systems.
Primary versus secondary treatment
Primary treatment can be best accomplished in a large communal septic tank equipped with effluent outlet filters, or an Imhoff tank. Although the Imhoff tank is more expensive to construct due to its two-tiered design, it provides a better and more reliable effluent quality than a large septic tank, and is more economical to operate because of its capacity to hold sludge and decrease its bulk via digestion. The septic tank requires more frequent de-sludging and produces an offensive watery sludge compared to the consolidated and stabilised solids removed from the lower digestion compartment of the Imhoff tank.
Secondary treatment of the dissolved and suspended organic waste matter in the settled effluent from the primary treatment process can be provided via a range of treatment options (see Table 8.4). These are discussed below.
Biofilter systems
These provide suitable secondary treatment for communities with a relatively constant population to maintain uniform loading and reliable treated effluent quality. All biofilter systems incorporate a 'secondary' settling tank to capture the biological sludges that accumulate in the system.
Media trickling filters are tanks of uniform-size gravel or crushed rock, or plastic-spoked wheels (or other plastic shapes, including corrugated sheets), on which grow the aerobic bacterial slimes responsible for cleansing the settled wastewater, and through which air circulates continuously as settled effluent trickles slowly down through stone or plastic media. The slime growths slough from the system continuously, forming a biological sludge for collection and removal from the secondary settling tank.
Rotating biological contactors (RBC) consist of 2–3-metre-diameter thin plastic discs 80–100 mm spaced on a rotating axle and turned slowly through a 'trough' of settled wastewater, so that the bottom third is continually being submerged. The intermittent submergence in wastewater and then exposure to the air creates aerobic bacterial slime growth on the plastic surfaces in the same way as the media filter described above.
Rotating drum biological contactors provide for biosolids growth on the internal media surfaces of the drum unit.
Activated sludge systems
Suspended growths of aerobic bacterial slimes are maintained by aerating the wastewater and suspended solids mixture by either bubble aeration or mechanical mix aeration. The wash-out of active suspended solids is captured in a 'secondary' settling tank and recycled back into the activated sludge tank to continue cleansing the incoming wastewater. Activated sludge variations can provide either 'secondary' treatment to pre-settled 'primary' wastewater flows, or full treatment of raw wastewater by what is termed 'extended aeration'.
'Package' plants are factory-assembled activated sludge-treatment units, ranging from single household size up to village size, which generally operate on the extended aeration basis. They can be readily transported from factory to site, set up on a concrete slab, and, after connection to an inlet sewer and power supply, can be operating within hours of arrival.
Sequencing batch reactors (SBRs) are a fill-and-draw system, which provides a simplified and economical alternative to the conventional extended-aeration activated-sludge approach. They can be operated to strip nitrogen nutrients from waste flows and hence are well suited to residential areas in sensitive environments. The Lake Taupo basin was the first application of SBRs for a small community in New Zealand.
Oxidation ditches are an extended-aeration activated-sludge system which uses a shallow oval, race-track-shaped aeration basin aerated by a surface mechanical aerator, which also maintains a steady circulation of mixed flow in the channel. Overflows are settled to produce a final effluent and sludge, which is recycled to the plant inlet. Excess sludge biosolids are removed periodically.
Aerated lagoons are a low-cost alternative to the extended-aeration activated-sludge system suitable for larger small communities. In some cases they can provide pre-treatment prior to oxidation pond systems. They have particular application in New Zealand holiday area communities, where during winter they operate as a simple oxidation pond followed by 'polishing' treatment in the accompanying oxidation pond. In summer the system is changed back to an aerated lagoon/oxidation pond configuration by activating the aerators.
Sand-filter systems
Packed bed biological reactors, or sand-filter systems, use sand or packed media (eg, crushed glass) to provide surfaces for bacterial growth, and voids for air circulation, bacterial storage, and physical straining. These systems can cope well with variable population loading rates.
Intermittent sand filters are used as secondary treatment following community septic tank or Imhoff tank pre-treatment. They can cope with fluctuating loadings more effectively than biofilter and activated-sludge systems, and produce a much better effluent quality. They also reduce human intestinal bacteria numbers (measured by coliform indicator organisms), as well as significantly reducing organic matter and suspended solids. They must always be preceded by primary treatment.
Recirculating sand filters are more economical to construct than the intermittent types because of their reduced size, but pumping costs for dose loading are higher due to the recirculation process. Recirculating textile filters replace the sand by a synthetic woven fabric, resulting in a very compact treatment unit with high performance in organic matter and suspended solids removal, but are not as effective at bacterial removal.
Oxidation ponds
Facultative ponds are the most common full-treatment system in use in New Zealand. The aerobic liquid depth fosters waste stabilisation via an algal–bacterial symbiosis, which matures incoming flow during a four- to six-week retention period. The anaerobic sludge layer on the floor of the shallow pond stabilises and consolidates settled sludges and algal cells. Pond systems can accept widely varying input loadings due to the buffering action of their considerable storage volume and detention time.
'Polishing' ponds (tertiary treatment systems) are usually sized on a 21-day retention time at average daily flow to allow algal solids from facultative ponds to settle, and human intestinal bacteria to die off before discharge of effluent. Some polishing or 'maturation' ponds consist of several cells in parallel, each cell with 5 to 10 days' retention capacity. The cells-in-series configuration improves the efficiency of bacterial removal. Maturation ponds can provide tertiary treatment for effluent from any type of secondary treatment system.
Constructed wetland treatment
Wetland systems are of two types: surface flow and sub-surface flow. Because the sub-surface flow units involve effluent treatment via flow through a porous 'soil' granular medium, some (but not all) Māori iwi accept that this meets their cultural objectives in handling human waste via 'soil' treatment before the resulting water flow enters natural water. The treatment performance of wetland systems is nowhere near as predictable as other treatment systems discussed above, and many wetlands are used as an environmental buffer treatment stage placed between the main treatment system and a receiving water into which a point discharge is made.
Surface-flow wetlands provide either secondary or tertiary treatment over a 5 to 10-day flow-through (retention) period. Emergent wetland plants that are rooted in the soil on the base of the shallow pond in which they have been planted work well, through settling and bacterial growth on plant stems, as well as aeration of the water by oxygen transfer processes. Septic tank effluent, oxidation pond effluent or effluent from secondary treatment processes can be treated.
Sub-surface flow gravel-bed wetlands are increasingly being used to provide a further tertiary treatment stage for facultative oxidation pond effluent flows. They are also used for combined secondary and tertiary treatment of septic tank or other primary effluent in smaller communities.
Overland flow
Overland flow offers both a treatment function and an ecosystem re-entry role. Treatment occurs within the topsoil mantle. To ensure that the aerobic renovation capacity of the soil is maintained, alternating cycles of load and rest are required (as is the case for 'rapid infiltration'). Effluent to be treated is spread over the upper surface of a sloping, grassed plot and is treated via sheet flow as it moves down to a collection system at the lower edge of the plot. As the wastewater flows over the land, some will be infiltrated into the soil, achieving re-entry to the ecosystem. Flow that does not soak in is collected as 'polished' effluent for appropriate disposal.
