THE "AED 10+" MINI SEWAGE TREATMENT PLANT
Introduction:
For several centuries, a septic tank coupled to a French drain soak-away has been the only choice of sewage and wastewater treatment and disposal in areas not connected to a municipal sewerage system. This has worked fine in many areas for many years. However, an increase in the incidence of groundwater contamination and restrictions on the use of French drains within certain municipal areas and in regions located on certain rock types prone to the development of sinkholes (such as dolomite) and in certain other environmentally sensitive areas, has prompted us to look at more modern alternatives to the conventional septic tank / French drain combination. Furthermore, the shortage of water in many regions in Africa has made it necessary to re-use water as far as practical.
Background:
Some
years ago we occupied our new premises on the farm Sterkfontein 173 IQ . This
farm is located on the Zwartkrans dolomitic groundwater compartment and also
falls within the Cradle of Humankind World Heritage Site. Being water
specialists, we immediately realised that a normal septic tank/French drain
would not suffice on dolomite, as a real danger exists of both groundwater
contamination and sinkhole formation where any soak-away type wastewater
disposal system is concerned.
We
therefore purchased a mini sewage plant and installed it on our premises. Over
time, we assisted in the development and improvement of this sewage system and
re-designed many of the shortcomings the original system had. We also acted as
agents for the manufacturer of the plant and sold and installed a number of the
units in and around the Cradle of Humankind World Heritage Site. Most of these
units are still fully functional and serving the households where they are
installed, well. In fact, they were such an improvement on the conventional
septic tank sewage treatment technology that the Department of Water Affairs and
Forestry (DWAF) provided us with a letter of support for this type of sewage
treatment technology (Read more about what DWAF said...).
Almost
all modern sewage plants operate on the same basic principles, be it large
municipal sewage plants or small mini sewage plants. Firstly, sewage is broken
down by employing the actions of anaerobic bacteria. These micro-organisms break
down some of the organic load in the sewage in the absence of oxygen, i.e. under
anaerobic conditions.
The
effluent from this process is then treated in a further section of the plant,
this time using aerobic bacteria. These bacteria require oxygen and this oxygen
is supplied into the water similar to the way oxygen is supplied to an aquarium,
i.e. air is forced into the water using various mechanical means such as a
compressor and an air diffuser.
In
most systems particular bacterial groups are used. Either the bacteria grow on
some form of artificial media, or they clump together and form flocks floating
free in the sewage being treated (The mixture of bacterial flocks and sewage is
referred to as mixed liquor). These bacterial flocks are then separated from the
rest of the mixed liquor through gravity in a settling tank. The bacteria are
returned to treat new sewage, while the (clean) supernatant water is disinfected
and becomes the effluent water that could be utilised in various ways, usually
in the garden.
As the bacteria break down the organic load in the sewage, they grow (multiply). Eventually a stage is reached where there are too many of them. In large municipal sewage plants these surplus bacteria are pumped into sludge drying beds or are irrigated onto pastures, instant lawn farms, composted or disposed of using various other methods. In smaller sewage plants such as our AED 10+ Mini Sewage Plant, they are pumped back to the anaerobic section of the plant where the anaerobic bacteria “consume” the aerobic bacteria producing carbon dioxide and some methane gas. These gases are then vented off into the atmosphere through the household’s sewage ventilation system.
Due to the small size, most mini sewage plants have problems in their design that cannot be engineered out. The size and shape of the separating tank is the number one fatal flaw in many designs. Under normal circumstances, a separating tank is a cylindrical structure with either a flat or conical bottom. Bacterial flocks would sink to the bottom and would then be removed by pumping them from the bottom of the tank. The biggest problem with most mini sewage systems, however, is the small volume of the settling tank. The settling tank should be designed in such a way that the speed at which clean water is drawn up and out of the tank does not exceed the rate at which the bacterial flocks sink to the bottom of the tank. If this occurs, the bacteria will be lost with the effluent. In large municipal sewage plants the flow through the settling tanks can be balanced and kept to an acceptable flow rate. However, in most mini sewage plants, the design dictates that "what goes in must go out". This occurs during peak flow conditions such as during the few minutes it takes to drain a bathtub. The netto result is that most plants experienced bacterial flock carry over into the effluent resulting in a too low bacterial population in the aerobic section of the plant, in turn, resulting in poor quality effluent.
Above: An almost-completed plant at a new homestead in the Cradle of Humankind World Heritage Site. It still requires a lid to cover the second stage in the foreground. A small manhole in this lid will provide access to the chlorinator.
Our Re-designed Plant:
Of
all the flaws in small sewage systems, the flaw of occasional hydraulic
overloading (such as when one or two bath tubs are emptied at the same time) is
the single largest problem. In municipal sewage systems, peaks are usually
spread over longer periods, as all sewage producers are not the same distance
for the treatment plant. They also employ balancing tanks to even out peak
flows.
In a
mini sewage treatment plant there are no balancing facilities, simply as the
plant is too small. Various methods of throttling have been attempted in small
treatment plants, but, when working with sewage and orifices, the latter become
blocked very rapidly by bacterial growth if they are too small. If we made
the settling tank large enough to accommodate the maximum expected flow, the
tank would have to be disproportionally large, which, in turn, defeats the
objectives of mini sewage plants.
We therefore designed the settling tank to be part of the aerobic reactor. Instead of first treating the sewage in an aerobic reactor and then separating the purified effluent water from the bacterial flocks in a separate process, we combined the two processes in one tank. Now we were able to use the entire volume for both the aerobic reactor as well as for the separating tank.
For the largest part
of the day, the tank acts as an aerobic reactor. Air is pumped into the water
and the aerobic bacteria remove the organic load from the water and turn ammonia
and nitrites into less-toxic nitrates. During this period, no effluent is
discharged from the tank. During a carefully chosen period at night (usually
around 01:00 when there is little or no new sewage entering the treatment
plant), a timer turns off the compressor that pumps the air into the mixed
liquor and the mixed liquor is left in quiet conditions for a period of
approximately 1½ to 2 hours. This provides sufficient time for the heavy
bacterial flocks to settle to the bottom of the tank. During this period, the
tank effectively becomes a settling tank. After the 1½- to 2-hour time period,
a pump is activated which removes supernatant water from the surface of the
water body in the tank through our uniquely designed floating skimming device.
When the water level drops to a pre-determined level, the pump stops and the
aeration again commences for another cycle. As an added benefit of the regular
cycling between aeration and settling, this process also effects the
denitrification process, where the nitrates, previously converted from ammonia
and nitrites, are converted to nitrogen gas. This gas is vented off into the
atmosphere.
Above: The general layout of the Aerobic Reactor/Settling tank section of the AED 10+ Mini Sewage Plant.
Above: A photograph of the internal components of one of the installed AED 10+ units. This unit is ready to be commissioned and has been filled with clear water. Once operational, the parts under the water surface will not be visible.
Disinfection:
The disinfection section of the AED 10+ Mini Sewage Plant comprises of a chlorinator that uses a cartridge of chlorine tablets. The type
of chlorinator used on our AED 10+ Mini Sewage Treatment Plant is
simple to use and to adjust. The grey top of the chlorinator is simply removed;
the old chlorine cartridge is removed and replaced with a new one. To adjust the
amount of chlorine dosed, the grey cap is merely screwed in deeper. Calibration
marks on the side of the chlorinator indicates the dosage. One cartridge of
chlorine tablets should last between 2 and 6 months, depending on the number of
people occupying the house/premises.
Our plant is Modular:
We have designed our AED 10+ Mini Sewage Plant in such a way that the entire plant is made with "off-the-shelf" products. This means that spares, etc. will always be available.
We have also designed it in such a manner that it is possible to perform all maintenance and repairs through the manhole in the second tank. This means that it will never be necessary to drain the tank for maintenance purposes.
Furthermore, as we use "off-the-shelf" products, we are able to design the plant to cater for any number of people (within the limitations of Mini Sewage Plants, of course). Our standard AED 10+ can handle the sewage from a residence with a maximum number of 10 people, including the servants living within the catchment of the sewage system. This is calculated at a hydraulic load of 200 l/person/day to sewer and a biodegradable organic load of 64 g BOD/person/day.
We can manufactured plants for larger installations such as lodges, conference centres or factories. Plants can be constructed in the ground (preferably) or on top of the ground, depending on the fall to the plant.
Above: An AED 10+ Mini Sewage Plant designed for the treatment of sewage of a larger number of people and is constructed above ground surface.
What
to do with the effluent?
Although the effluent is not purified to a drinking standard, it is
nevertheless safe and may be used in a variety of applications. There are many
ways the final effluent produced by the AED 10+ Mini Sewage Treatment Plant
may be utilised.
The most useful application of the effluent is to irrigate it onto lawns,
gardens or crops. In this case, effluent will have to be stored in some form of
reservoir until enough effluent has accumulated for irrigation. Please also
adhere to the safety principles discussed hereunder.
A plastic-lined reservoir is the simplest method of storing
effluent water produced by the AED 10+ Mini Sewage Treatment Plant. It
can also be turned into an attractive water feature by installing a water
fountain in the reservoir. When sufficient water has been accumulated, a
centrifugal pump may be employed to spray the effluent onto gardens, etc. If
enough of a slope is present, the water could be gravitated onto crops
intended for irrigation.
A second alternative to storage is to direct the effluent into an
artificial wetland. This wetland could ultimately discharge into a pond or it
could be constructed in such a manner that it would use all the water within the
wetland through evapotranspiration. Please always keep the environmental
considerations, discussed hereunder, in mind when designing and constructing
such a wetland.
There are many other industrial and domestic uses for sewage effluent not discussed here.
Environmental Management and Safety Considerations
Although the AED 10+ Mini Sewage Treatment Plant
is intended to replace the septic tank / French drain entirely in all areas not
serviced by a municipal sewer, it was primarily designed to be used in
environmentally sensitive areas or in areas where the geology is such that
septic tanks / French drains are not permitted, i.e. where groundwater
contamination or sinkhole formation may result from French drains. As described
above, an anaerobic septic tank does not purify sewage water to nearly the same
quality as an aerobic process.
The following items are therefore listed as an
environmental/safety guide when installing or operating the AED 10+ Mini
Sewage Treatment Plant.
Safety Considerations
· Never drink the treated effluent. It has not been treated to a drinking water standard. Mark all taps with effluent water with a clear “non-drinking water” sign.
· Always treat raw sewage water with respect. It may contain pathogenic bacteria (bacteria that could transmit diseases to people coming into contact with the water). Always wear protective clothing and gloves and wash your hands with disinfectant soap immediately after handling any untreated sewage or sanitary fittings.
· Although chlorine is one of the most efficient disinfectants used in the water treatment industry, do not assume that the effluent is safe. The chlorinator may not be adjusted correctly. Always treat effluent as if it may contain pathogenic bacteria.
· When irrigating the effluent, never allow it to be sprayed directly onto people or animals.
· Do not irrigate treated effluent onto vegetables or fruit that are normally eaten raw.
· When irrigated onto the lawns of sports fields, always allow for at least two days of drying in the sun before any sports activities are allowed on the field, especially in the case of sports where people may fall and come into direct contact with the lawn.
· Adhere to open dam safety principles, especially when there are children around that cannot swim. Always protect all open dams with some type of barrier similar to that used around swimming pools.
·
When there are children around, ensure that the manhole lids of
the tanks are locked into place in such a way that they cannot be opened by
children.
If a person falls into one of the units drowning could result.
Environmental Considerations
· Never allow effluent to be discharged directly onto ground near a drinking water borehole. This is of particular importance if the underlying geology is dolomite. As a rule of thumb, provide for a distance of at least 100m from any borehole. In dolomite, however, there may be preferential access routes directly into the groundwater from much further distances. Also keep the distance from your neighbours’ boreholes in mind!
· If effluent water is discharged into an artificial wetland, always ensure that the wetland is constructed on impermeable soil or alternatively, construct such a wetland using an impermeable lining.
· Never allow water to stand in a permanent un-lined pond in areas underlain with dolomite. This practice may lead to the formation of sinkholes and could also contaminate the groundwater.
· Ensure that there are no leaking pipes/couplings, especially pipes containing raw sewage en route to the sewage treatment plant. This may lead to groundwater contamination.
· Do not discharge the effluent produced by any sewage treatment plant directly into any public stream without a water license from the Department of Water Affairs and Forestry.
·
Under normal circumstances, a sand trap or grease trap is not
fitted ahead of the plant. Under conditions where there may be excessive grease
or sand/gravel entering the plant, it is recommended to install these in order
to protect the plant from premature siltation.
The AED Mini Sewage Treatment Plant is designed to treat the sewage produced by a single household of 10 people calculated at a hydraulic volume of 200 l per person per day and a Biochemical Oxygen Demand (BOD – measure of organic load) of 64 g/person/day.
If used within its design criteria the effluent produced by the plant will meet the General Effluent Standards (excluding the Special phosphate standard).
THE "GEM" MINI SEWAGE
TREATMENT PLANT
We no longer market the GEM Mini Sewage Plant. However, for our customers who have installed these units, we still offer backup in the form of the following downloads:
Download the Technical Document in MS Word Format...
Afrikaans Version English Version
Download the Operating and Installation Guide
Installation and Operating Instructions (English version only)
(600 KB - This may take a few minutes)
"GEM" Maxi Sewage Plant (English Only)
Click on any of these hyperlinks to download the document...
