Septic tanks

SEPTIC TANKS and other ON-SITE SYSTEMS

For over a quarter of a million homes in Australia, the lack of a municipal sewer connection doesn't mean that appropriate health controls of human and domestic wastes cannot be effective in minimising pollution and avoiding off-site health impacts. In NSW, the regulations are quite clear as to the performance criteria that will be enforced by local councils when inspecting on-site systems (see NSW Local Government (General) Regulations 2005). Similar legislation in other states reflects the long held view that human waste is a potential source of wider health and environmental risks. Minimising those risks is the key to effective and economic on-site treatment systems.

So what systems are available for on-site domestic wastewater treatment?
Well, any device or process that can treat human wastes to meet the performance criteria to minimise health or environment risk to either the occupiers or neighbours. A simple "long drop" (pit latrine) may be all that is required for a church hall where a small community meets once a week, or a highly engineered system may be required for a community housing estate on the unsewered fringes of a major city. Many examples exist of housing estates where the residents have a 'community title' to a packaged treatment system and the community owns, manages and operates the scheme to meet the licensed conditions. For systems over 20 persons equivalent, licensing by a government agency is required and on-going monitoring and conformance are stipulated in the licence. For single systems, the local authority usually provides a 'licence to operate an on-site sewage treatment system', although in Victoria, the Environment Protection Agency (VicEPA) is the sole responsible agency. Other agencies, such as Health Departments may be involved.

The sections below are by no means comprehensive. Between 1999 and 2007, Lanfax Labs coordinated five three-day conferences on On-site Systems at which more than 250 professional papers and keynote addresses covered a variety of topics from compost toilets to engineered systems. The tables of contents for each of those conferences can be viewed under 'publications'. At some time the individual papers will be available on this site.

Now for some of the simple detail for on-site systems. Note that when doing a word search on the internet, the Americans avoid the hyphen and refer to 'onsite' as a whole word. The Australian Macquarie Dictionary refers to 'on-site'.

I should clear up my understanding of the words 'dispose' and 'disposal'. Some politically correct people think that 'disposal' is not appropriate and 'dispersal' includes the re-use of effluent for irrigation. So be it! I will use 'disposal' to also include re-use because whether we like it or not, our aim is to get rid of the water through the hydrologic cycle - mostly as evapotranspiration - and that is disposal. That we also include growing of plants doesn't negate the fact that we want to get rid of the water as safely as we can - that's disposal.

On-site system layout

The figure to the left shows the essential elements of an on-site wastewater management system (OWMS). Yes, the tennis court and garage are important parts because they prevent those areas being used for effluent disposal.

The house: generation of wastes from toilet (faecal, paper, urine, water), kitchen (greases, oils, food scraps, salts, detergents, soluble nutrients, hot and cold water), the bathroom and basin (body oils, bacteria, hair, personal care items, detergents, soaps, cremes) and the laundry (lint, detergents, mineral soil, greases). The water system may also provide salts, trace elements (including zinc and copper, chromium, nickel, lead) while bore water may also contain other minerals.

The occupants are responsible for the volume of water generated as wastewater and all the chemicals and physical additions to the water from activities within the home.

Water traps: all the devices connected to the wastewater will have a 'water trap' ('S' bend, 'P' bend) to prevent the return of gases from the waste pipes returning into the home.

Septic tank: a primary treatment system that operates in an anaerobic mode, that is with anaerobic bacteria that operate in an oxygen free environment. The septic tank provides a storage component during which period solids more dense than water settle to the bottom and light materials float to the surface to form a scum. Other treatment systems, such as aerated wastewater treatment systems may be required to upgrade the treatment process.

A dry system, such as a compost toilet or deep pit latrine ('long drop') can treat human wastes but may be less efficient at handling high urine loads.

Land application area: an area of land within the confines of the property boundaries in which the discharge from the septic tank is spread in such a manner that the water and its dissolved and suspended solids are absorbed into the soil profile for final treatment. The nutrients are absorbed by the vegetation or retained in the soil, the water drains slowly downwards and sideways under capillary flow and upwards to evaporate into the atmosphere. The harmful bacteria (pathogens) are destroyed within the soil by many processes.

Whether the land application system is by a series of trenches, by surface or subsurface irrigation or by evapotranspiration beds is a matter for the individual system based upon land constraints (these will be discussed further)..

The surface of the land application area must be protected from traffic (vehicle, animals and humans) as compaction when wet will impede the effective movement of water. Some areas may require diversion of potential run-on water away to other areas.

Property boundary: All wastewater treatment must occur within the property boundary and no liquid should enter the neighbouring lands or waterways, either over the surface or by sub-surface flow (that includes soggy areas through into the neighbour's property..

The treatment process (wet system)
1. Primary Treatment
Wastewater enters the septic tank from the left and flows below the scum layer that develops on the surface. The 'T" piece ensures that the inflowing waste enters below the surface. The wastewater (liquid with entrained solids) mixes with the liquid contents of the tank, the more dense particles settle to the bottom and the lighter particles float to the surface, becoming part of the surface scum seal. This scum on the surface is the most important part of the operation, as it excludes air from the liquid, allowing the anaerobic bacteria to thrive. In some areas, home owners are 'conned' by pump-out contractors into believing the tank is 'full' when there is a scum on the surface and hand over many dollars to have the tank evacuated unnecessarily. An effective septic tank will generally only require pump-out every three to seven years, depending upon vigilance of what goes into the system.

While held in the tank, the solids may separate into small parts (big lumps become small lumps), and some very small particles (colloids) may clump together and settle to the bottom, while other particles will stay suspended for long periods. For this reason, the volume of water in the tank should be at least the 24 hour volume of wastewater generation, after allowing for maximum sludge and scum formation.

The liquid portion is made up of water, dissolved components and suspended materials that don't have sufficient mass for their volume to sink, or light enough to float. Bacteria may start to decompose any of these three components (scum, liquid, sludge). Anaerobic bacteria live in this environment where oxygen is a poison. The oxygen that enters with the wastewater is quickly consumed and overall, the dissolved oxygen level will be close to zero. The discharge will, therefore, be anaerobic in which ammonia, hydrogen sulphide (rotten egg gas), methane and other putrid smell are common dissolved gases. Be aware! These gases are poisonous and some are explosive.

So, the big question is - Should a septic tank smell? While there are many obnoxious and harmful gases in an anaerobic septic tank, the scum layer usually minimises their release to the atmosphere, so where a solid, dry scum layer forms, the smells will be minor. When the scum layer is watery, broken or fails to form, smells will always be more noticeable.

A baffle may be used to form two compartments in the one tank, the first chamber being about 2/3 total volume and the second chamber the remaining 1/3. The baffle has holes below the scum layer to allow waste to pass slowly through from the first to the second chamber. The outlet is in the second chamber where the wastewater has been partly clarified. It is still very dirty water, smelly and with large populations of hundreds of species of bacteria, as well as viruses and parasite. Primary treatment does little to reduce the contaminated nature of the water. It is certainly not clean enough to swim in, and the thought of drinking it is revolting! Yuk!

Over years, the scum will accumulate to the point where, if allowed to continue, would be deep enough to overflow through the outlet. Similarly, if the sludge accumulates too much, it may be sucked out with the discharge. The primary tank is designed to limit the discharge to within reasonable limits. Therefore, at some time the septic tank will have to be de-sludged depending upon the rate of build-up of scum and sludge. Don't be conned by those who see the scum as a sign of the tank being full.

Typical discharge from the septic tank, now call effluent (because it has been through a treatment process) rather than wastewater (untreated), has about 50-200 mg/L total suspended solids (TSS), pH about 7.0-7.5, electrical conductivity (EC) about 0.7 -1.5 dS/m, biochemical oxygen demand (BOD₅) of around 250 mg/L and bacteria many times more than 100 000 colony forming units per 100 mL (cfu/100 mL). Because we cannot see bacteria without very strong microscopes, some bacteria are cultured under special conditions until they form colonies that are visible as dots on a filter. These colonies can be counted and reported as cfu/100 mL. Many thousands of species of bacteria live in a septic tank, but we only culture faecal coliforms because they are representative of the species that live in the gut of warm-blooded animals.

A septic tank is a biodegradation device that operates in a passive way; wastewater in equals effluent out without any external energy input.

What do we now know?
1. Primary treatment vessel must be a big tank that holds more wastewater than is generated in one day. The tank must be water-tight and resistant to chemical attach.
2. The tank operates passively in an anaerobic state, producing smelly gases under ideal conditions.
3. The scum is essential to keep the contents anaerobic and to minimise the escape of smelly gases.
4. After a period (perhaps 3-7 years) the tank will need to be pumped out to remove the accumulated sludge and scum.
5. Anti-bacterial products used in the home may be detrimental to the bacteria in the tank - those bacteria that are helping to do some primary treatment.
6. The discharge requires further treatment and needs to be kept away from humans, animals and the wider environment.

WARNING: Everything you put down the sink, toilet, shower or laundry will end up in the septic tank and harsh chemicals may seriously reduce the capacity of the primary treatment system or worse - render the discharge unsuitable for on-site application.

Septic tanks can operate on sea-water, so salts from detergents and soaps are unlikely to affect the operation of the tank, but high salt levels may impinge upon the ability of the soil to adequately move the water through the soil for its final treatment. Irrespective of advertising by manufacturers of detergents, toilet paper and other household chemicals, there is no guideline or standard in any state of Australia for septic safe products. Manufacturers who label products as "safe in septic' are trying to 'pull the wool over your eyes'. It may be safe in the septic but it may be disastrous to the soil - so your system will fail if it cannot effectively deal with the water and nutrients.

For many on-site systems, the septic tank is the end of the engineered treatment system with the effluent discharged to drainfields in the land application area. Unfortunately, many home owners think that the septic is the end of the matter. Septic tanks are simple treatment devices - so simple they can hardly go wrong. But the soil is the most complex biological system on earth. The interface where living and non-living components live. So if the soil system fails, your on-site wastewater management system has failed, and a simple solution such as pumping out the septic tank will NOT solve the problem.

2. Secondary Treatment
In some situations, where the discharge of primary treated effluent to the soil would require large land application areas, or where the subsoil is unsuitable for disposal of effluent, it may be necessary to take the next step in the effluent clean-up through another engineered device, such as an aeration device, a wetland, a sand filter or biofilter. Secondary treatment removes more of the total suspended solids (TSS), reduces the biochemical oxygen demand (BOD) and eliminates a large proportion of the bacteria. Some secondary treatment system claim much more, but under real conditions few meet those goals.

The secondary process involves the provision of a medium for entrapping suspended solids (reducing the time it takes for solids to settle onto a firm surface) and convert the environment from anaerobic (without oxygen) to aerobic (with oxygen) because aerobes operate 10 times faster than anaerobes and the by-products of degradation are carbon dioxide and not smelly odours. These two processes reduce the total suspended solids and the biochemical oxygen demand, and convert ammonia to nitrate as well as eliminating other odours.

Process 1: Fill the receiving chamber with a honey-comb medium that provides surfaces on which solids may settle.

Process 2: Use an electrical-mechanical device to pump oxygen into a diffusing system to provide very small air bubbles over the honey-comb medium (inject in the bottom).

Process 3: Pass the aerated effluent into a clarification chamber where solid particles may flocculate and settle by suspension. Pass clarified liquid to irrigation tank, return sludge to primary tank (septic tank).

Process 4: Capture the clarified liquid in another chamber from which it can be irrigated.

Between Processes 3 and 4 there may be a disinfection stage. Effluent passes over tablets that release chlorine into the effluent stream, thereby killing some of the bacteria and leaving a residual dose of chlorine. This residual dose can be measured and reported.

Be AWARE!. Chlorine also kills friendly bacteria in the soil. While Health Officials feel comfortable with the use of chlorine, it does not kill all parasites and bacteria species - just some.

3. Land Application Area
Many on-site systems fail in the land application area without the knowledge of the home occupier because it happens away from the house, often in an area unused by the family, left overgrown, or ignored because its always wet under foot. Many home owners do not even know where the land application area is located, whether it is one or a series of trenches, how long the trenches are and whether the effluent is actually treated in the soil or simply bubbles out at the surface. So should we know? And the answer is YES!.

Even though, at some stage in the past, an application was made with your local council to install the system and they had to check it before the trenches were infilled (in NSW this was the law as far back as 1950), don't expect the council will still have any plans. The new laws pass all responsibility to the land owner. So there is a need-to-know!

The land application area is the final treatment mechanism for on-site wastewater management. Here the system will be judged as a success or a failure. An over-filled septic tank with blocked outlets is easily rectified with a pump-out, but a failure in the land application area can be much more expensive to fix.

The first principle is to identify the type of land application system and then seek to understand whether the soil is adequately treating the effluent. Is the method of disposal through one or a series of trenches? How are the trenches loaded with effluent? Are the trenches along the contour? Are there wet spots along the trenches? Is the application method subsurface irrigation? So let's look at some of the traditional disposal methods in the next sections.

Traditional trenches (drainfield or soakaways)
Trenches are constructed using a back-hoe, fitted with a corrugated half-circular plastic arch, backfilled with gravel and finished with surface soil. The figure below shows the cross-section of such a trench. It is important that the trench is shallow rather than deep to encourage the movement of water up to the root zone from where it is lost by evapotranspiration by the plants and soil. Because many Australian soil profiles have clay subsoils, it is important to avoid these clays because they have very low permeability resulting in very long trenches, or a series of many shorter trenches.

Note that the arched trenching is backfilled with gravel and covered with a geotextile membrane prior to backfilling with top soil. During excavation, the subsoil (clay) needs to be separated from the topsoil and not reused in the trench. The clay may be used to construct a small bank upslope of the trench to direct runoff away from the land application area.

The benefit of the trench system is that it is passive - requires no energy and very little maintenance over decades, other than mowing the grass on the surface.

The top of the trench is shaped to displace rainfall and will need to be reshaped after the soil has consolidated months after completion. Sow the surface with a locally suitable lawn mix. Do not plant trees on the trench, but trees and shrubs can be place away from the edges of the trench.

Do not grow vegetable over the top of the trenches.

In rural and semi-rural areas, a fence may be required around the land application area to prevent stock bogging the area after rainfall when the soil will be wetter than the surrounding area.

If depressions occur in the top of the trench, backfill these until the soil has a mounded shape and ponding of water does not occur.

Trenches are contructed along the contour (parallel to the contour) so that water will form at equal depth along the length of the trench.

In some areas, arched trenching is unsuitable because the wet soil cannot support the load bearing edges of the arch. In these area, the arch trenching is replaced by slotted pipe that will distribute the water throughout the trench as shown in the figure below. The same principles apply as outlined above.

Multiple trenches (series or parallel)
Where the calculated length of trench exceeds 25 m, it is preferable to construct several trenches, each not more than 20-25 m long, either in series (left hand drawing) or in parallel (right hand drawing). The trenches are stepped down the contour so that each trench is parallel to the contour. It is important that the trenches suit the topography rather than be neatly set out parallel to each other.

The benefit of the parallel trench layout is that each trench operates with a proportion of the effluent through the distribution box. However, the distribution box needs to be accurately sited and levelled and its performance regularly checked and the distribution reset if required.

In a series layout, the first trench is always wet and only when one trench is saturated will it overflow (through a pipe) to the next trench and so on through the series. The least work is done by the bottom trench.

Trench failures
Trenches fail for many reasons, not the least is a high solids carry-over from an overloaded septic tank. Incorrectly sizing the drainfield according to the permeability of the soil, the loading rate and local climatic variables may all contribute to trench failure. In many cases, adding an extra trench to the system may suffice. It is usually uneconomic to dig up a failed trench to rebuild it, because the failure is in the soil around the trench and not in the trench itself. Construct new trenches elsewhere.

Irrigation of Effluent
Surface irrigation of effluent from a primary treatment system is hazardous because of the extremely high faecal bacteria and potential pathogens in human sewage, not that human sewage is any worse than animal faecal material, it's just that primary discharge is also septic and specifically human. Secondary treatment systems usually include a disinfection step between the clarification chamber and the irrigation storage so that bacteria numbers are significant reduced from hundreds of thousands to hundreds (that's certainly the aim).

Two real problems with surface irrigation include the control of surface ponding from irrigating at a rate greater than the infiltration capacity of the soil and the likely contamination when the irrigation system sprays during rain and runoff is contaminated and enters the wider environment without adequate control.

Sub-surface drip irrigation (SSDI)

Special wastewater drip irrigation pipe must be used for all sub-surface drip lines. Ordinary irrigation drip line is unsuitable as the inline drippers have not been designed for wastewater which is not as clean as normal irrigation water. Wastewater irrigation line is purple.

In the figure to the left, a typical layout of a drip line is shown. Essential are the filter and the return flush line.

Two valuable websites for wastewater drip irrigation are: Netafim and Triangle Waterquip.

Sequential irrigation
The maximum benefit of irrigation, surface or subsurface, is gained by irrigating an area until the water deficit is met and then allowing the soil to return to a drier state. Field capacity is a term used to describe the moisture content of the soil after it has been saturated and then allow to freely drain, in other words there is no visible water on or in the soil. IOrrigation should bring the soil up to field capacity and then wait until the soil has dried considerably, but not so the plants are under stress. Such an irrigation strategy is difficult in a domestic situation so a sequential irrigation value is used.

The sequencing valve opens to one port at a time. When the pump finishes and the pressure drops, a spring returns the port outlet to the next port in the sequence.

The benefit of this system, as shown in the figure to the left, is that an area is irrigated only once in the sequence and has time to drain before the next cycle. How much water is delivered on each cycle depends upon the setup in the irrigation tank and the volume of water available for pumping.