Sewage Treatment

Articles Tagged with: Sewage Treatment

13 January, 2014

Arkitrek Septic Tank Design

The following drawing is for a brick septic tank, suitable for Population Equivalents (PE) up to 50 people. When calculating your PE you must take the worst case scenario i.e. the maximum number of people who will be contributing to the facility. Remember, a septic tank should always be considered as only the first stage in the sewage treatment process. For a detailed explanation of how septic tanks work in a sewage treatment system, please refer to our highly popular web page In The Shit: A Septic Tank Guide

Arkitrek Septic Tank Guide Open Source Design Drawings

Arkitrek Septic Tank by Arkitrek is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Based on a work at http:/arkitrek.com/arkitrek-septic-tank-design/.

This is a standard design that we have developed with input from engineers and used on many of our projects. We are however architects, not engineers, and no design liability can be accepted for either Civil and Structural Engineering design or Mechanical and Chemical Engineering design i.e. that the tank will hold up, that sewage will flow and that effluent will meet a given standard. If you have any technical questions we will be happy to refer you to a friendly engineer.

7 February, 2012

In the Shit – A Septic Tank Guide

This post has been updated. The original ‘in the shit, a septic tank guide’ has been archived .

Two common misconceptions about septic tanks.
1. Septic tanks are an acceptable sewage treatment solution
2. Septic tanks are not supposed to overflow

The first misconception is the biggy; A septic tank on its own is not good enough to treat sewage waste to an acceptable standard. Further treatment is required. The second misconception shows a basic misunderstanding of the processes at work in a septic tank.

This article illustrates what happens inside a septic tank, what should happen after the septic tank and appropriate effluent standards to be aiming for. To build your own brick septic tank you can download our design for free

Septic Tank Processes

The following illustration shows a cross section through a typical off-the-shelf plastic septic tank under normal operating conditions.

Septic tanks operate by displacement and do not normally require pumps. Each time a new ‘delivery’ arrives at the influent (inflow) pipe it will displace an equal volume of liquor at the effluent pipe. Septic tanks should be filled up with water when they are commissioned and thereafter will continuously ‘overflow’ as long as there is raw sewage going into them. When I say ‘overflow’ I mean liquor will flow out of the discharge pipe, not that liquor will overflow from the inspection hatches. We’ll come back to this in a bit in the ‘de-sludging’ section.

Septic tanks should be designed with a retention time of at least 24 hours. i.e. it takes 24 hours for liquor to pass through the tank. The size of the septic tank should therefore be equal to or greater than the total volume of sewage waste produced in 24 hours.

This 24hour retention time is enough time for the sewage waste to separate into three constituent parts: sludge, liquor and scum. 24 hours is not long enough to treat the sewage waste to an acceptable standard. We’ll come back to what is an acceptable standard in a minute.

The Sludge

Most solids will settle to the bottom of a septic tank where they will slowly accumulate forming a liquid sludge. A baffle on the inlet pipe will slow down new deliveries so that they don’t disturb the settling process and also limit short-cutting.

The Scum

Mostly fats, oils and grease (FOG) which are less dense than water so they float. Scum can form a fairly hard crust which will slowly be digested by bacteria in the septic tank. Baffles on the inlet and outlet pipes help to prevent them being blocked by scum.

The Liquor

This is not clear water but nor is it raw sewage so it is called liquor for want of a better word. The level of the entrance to the outlet pipe or chamber should be within the liquor layer so that neither sludge nor scum leave the septic tank.

De-Sludging

Sludge will get deeper and eventually need to be removed. This is usually done by a ‘honey dipper’ as the Americans euphemistically call it. In reality it’s a tanker truck with a big dirty vacuum cleaner. Sludge pumped out into a tanker can be removed to a municipal sewage treatment works. In Malaysia there is no guarantee that this is an effective solution and we believe that architects and engineers have a duty to verify the performance of the local municipal treatment plant before choosing this option as the most responsible way to discharge sewage waste.

On sites without road access the sludge can be pumped out using a mobile pump. The sludge should be stored in an adjacent tank for 6 months to digest, after which lime can be added to raise the pH which kills any remaining pathogens. This process is called stabilisation. After stabilisation sludge can be dried on a drying bed and used for fertilizer. Stabilised sludge can also be used directly as liquid fertilizer.

The above illustration shows what happens if sludge is not cleaned out, but instead allowed to reach the outflow level of the effluent pipe. Any new delivery will cause sludge to be displaced as effluent. Clearly this is not good. In some cases the effluent pipe will become blocked and scum followed by sludge will come out through the inspection hatch or back up into the building. Clearly this is not good either. It is usually fairly obvious when this starts to happen because of the smell.

Perhaps more importantly, if sludge is allowed to flow out of the septic tank it may compromise the efficiency of the overall sewage treatment process. As we have seen at the beginning; a septic tank on its own does not treat waste to an acceptable standard. So what will?

After the Septic Tank

Here are the three sewage treatment options which we have tried in combination with a septic tank.

Aeration

By far the best option for sewage treatment is aeration. In a septic tank, digestion of the sewage waste takes place an-aerobically (without oxygen). Anaerobic digestion takes a long time, certainly much longer than the 24 hours retention time of a septic tank. If we were to rely on anaerobic digestion we would need very large tanks to provide the necessary retention time.

As soon as you introduce air into the sewage treatment process things speed up dramatically. This is because bacteria + oxygen are much more efficient at digesting sewage waste than bacteria without oxygen. With aeration, sewage waste can be acceptably treated with retention times of less than 24 hours. Tank size and overall plant space can be kept small.

An aeration system would typically use a septic tank to separate out sludge, liquor and scum as we have seen. The liquor will then be pumped or displaced through a series of two or more aeration tanks. The big advantage of this system is space saving and high quality of the finished effluent. The downside is that it is more expensive to set up and requires more maintenance than some low tech solutions.

Leach Field

A leach field is the preferred partner for septic tanks worldwide. The principle is to take the liquor from a septic tank and distribute it to a network of perforated pipes buried under the ground. The liquor will seep into the soil where bacteria will digest the organic waste and roots of plants will take up water, nutrients and minerals.

This system relies on the soil being porous but not too porous and also that the ground is not waterlogged. In Malaysia we have found that leach fields do not generally work due to the high rainfall and propensity for waterlogged ground. The converse is true of sandy areas like coral islands where effluent from a leach field would drain through the soil too quickly and enter water courses, aquifers or the sea before it has been adequately treated.

The advantage of a leach field is that it is low tech and cheap. The septic tank must be de-sludged regularly so that sludge does not overflow and block up the leach field but apart from that they are relatively maintenance free. The disadvantage is that they use up a lot of space and require suitable ground conditions.

The following photo shows a leach field being installed by Camps International in Cambodia.

Constructed Wetland

A constructed wetland is a series of ponds of varying depths with either an open or closed water surface. The latter is achieved by infilling the ponds with large grade gravel. Constructed wetlands rely on a mixture of aerobic and anaerobic digestion and the uptake of nutrients and minerals by plant growth.

The advantage of a constructed wetland is that plant species can be carefully selected to remove specific metal and mineral contaminants. They are also low tech and relatively maintenance free. The disadvantage is that they require a large area and are expensive to build, although not as expensive as an aeration system.

A drawback that we have found with constructed wetlands is that typical septic tank liquor is still far too nasty to discharge directly to a wetland. It smells and kills the plants. An intermediate treatment is required; perhaps a single small aeration tank, a large anaerobic digestion tank or an oxidation pond.

Composting Toilets

Composting toilets are an excellent way of treating toilet waste. In a tropical climate they quickly produce a neutral and odourless organic fertilizer. The drawback in Malaysia is mostly cultural; Malaysians like to use a lot of water in their toilet and water is the enemy of a composting toilet. With every deposit in a composting toilet there must be a corresponding addition of dry organic material such as saw dust or rice husk to soak up the excess liquid.

The other drawback of a composting toilet is that although they deal with piss and jobbies, you still have to treat grey water from your shower, laundry and kitchen waste. This grey water can be every bit as nasty as so called black water and must be treated as such. Soaps plus other fats, oils and grease (FOG) in particular must go through a sewage treatment process to digest them.

You can’t get away from the need to treat liquid sewage waste to an acceptable standard, whether it is black water or grey water.

Acceptable Standards for Sewage Treatment

Keeping it simple (because we’re architects not chemists) there are several things to measure in your sewage effluent but BOD and Suspended Solids (SS) are the two main ones for domestic waste.

Biological Oxygen Demand (BOD) is a measure of the potential for the effluent to remove oxygen from any water that it is discharged into. This is important because removing oxygen from water will kill other things such as fish, invertebrates and plants that might also need to remove oxygen from the water. An obvious visual sign of high BOD are algal blooms; coloured blue or green in fresh water and red in the sea.

Malaysia has two standards for treatment of sewage waste; Standard A is for discharge into drinking water catchment areas and Standard B for all other areas, including the sea. We believe that as a rule of thumb standard A should be the minimum target everywhere.

Standard A criteria are; BOD = 20mg/l @ 20ºC and SS = 50mg/l

A typical septic tank effluent is BOD = 118 to 189 mg/l (this is about a 40% reduction from the level of the entering sewage) and SS = 38 to 85 mg/l. source: Tomher Environmental A septic tank is therefore not likely to meet Standard A for BOD but might scrape through Standard A for SS.

Sewage also contains pathogens such as bacteria like E. coli or parasites and viruses. A septic tank achieves little or no reduction in the level of pathogens. Even treated sewage to Standard A or above will still contain pathogens. In sensitive areas such as bathing beaches and areas of fish production for human consumption, disinfection of the effluent after treatment may be necessary.

Conclusion

Septic tanks work well as a primary settlement tank at the start of a sewage treatment system. They do not treat sewage waste to what we deem to be an acceptable standard. To do this a septic tank must be combined with appropriate secondary treatment. Simply discharging septic tank effluent directly into a stream or sand near the beach is not good enough, although I have to confess, on some of our projects we are still doing this. There’s always room for improvement and the key message here is not to ignore the problem in the hope that it will go away!

To build your own brick septic tank you can download our design for free here

20 April, 2011

Futur Arc: Environmentally Responsible Sewage Treatments

Down To Earth Matters: Environmentally Responsible Sewage Treatments
The following article appeared in the 2011 1st Quarter edition of Futur Arc magazine
By Sarah Greenlees

We have treated the oceans as a “supermarket and a sewer;” Sylvia Earle, a prominent oceanographer and conservationist, was recently quoted to say in Time magazine [i]. As a result we put at risk the earth’s capacity to balance carbon dioxide and other gases in the atmosphere as well as destroying some of the least explored ecosystems.

A recent study into the world’s rivers led by Charles Vörösmarty of City Collage, New York (published in Nature magazine; 30 Sept 2010; “Rivers in Crisis”) revealed that 80% of the world’s population lives in areas where the rivers are in a threatened state [ii].

Architects striving to have a low impact on the land should also consider the impact of development on our seas and waterways. This makes treatment of sewage one of the most important factors of environmentally responsible design. In rapidly developing countries municipal sewage treatment plants are often oversubscribed and have trouble keeping up with the rate of development. Feeding into the municipal line in this case merely side steps the issue of responsible sewage management rather than finding a solution.

Architects Hasegawa & Xavier, of Malaysia are trying to reduce dependency on poor quality municipal infrastructure. One option is to treat sewage on site; however conventional systems such as extended aeration take around 24 hrs to digest waste, have a large footprint, require a fairly high level of maintenance and are prone to break downs resulting in unpleasant odour problems. Recently, a new product has been introduced in the South East Asia region by Tomher Environmental Sdn Bhd – the Enhanced Extended Aeration Processing System, or BioSolve.

The Infinity BioSolve system concentrates on providing optimum conditions for microbial life, the system is more efficient and is able to digest sewage in a quarter of the time of conventional systems. This means that it is a lot smaller. It is also flexible so that it can be designed specific to each site and brief and is far simpler than standard systems to run. Most importantly, it digests sewage to a higher level than current systems and removes virtually all pathogens.

There are two principal by-products of sewage treatment; water and de-watered sludge. In the Infinity BioSolve system, the water effluent typically has a Biochemical Oxygen Demand (BOD) of 5mg/ BOD and 15mg/l SS (suspended solids). This exceeds Australian and European standards, as well as those in South East Asia. BOD is the amount of oxygen required by aerobic micro-organisms to decompose the organic matter in a sample of water, such as that polluted by sewage. It is used as a measure of the degree of water pollution. Liquid sewage effluent can be treated further to remove nitrates and phosphates; however, these chemicals are ideal for irrigation and hydroponics systems.

The sludge from the Infinity BioSolve receives a higher level of digestion and can be turned out to drying beds where the sludge is made into fertilizer. Sludge drying and composting is unlikely to be practical on an urban site therefore it must be discharged to the municipal line. In this case an advantage of Infinity BioSolve is in the reduction in volume and toxicity of waste discharge to be handled by municipal infrastructure.

The flexibility of this system and its effectiveness make it an interesting product. While drying beds are not usually appropriate in an urban centre, hydroponics systems can be adapted to make use of the liquid effluent. Hasegawa & Xavier are currently proposing a vertical garden facade for a new urban project in Kota Kinabalu, Sabah, Malaysia. They also propose that the vertical garden be fed by a modification of the Infinity BioSolve hydroponics system.

The project has approximately 370m² of vertical garden facade. Usually this would be fed with large quantities of water and fertilisers. The proposal to use a hydroponics system linked to sewage treatment will avoid the use of fertiliser and will recycle water from the building, making savings on fresh water consumption. It will also greatly reduce the degree of reliance on municipal infrastructure.

Before the treated sewage is discharged to the municipal line, it is first sent to a balance chamber and drained of fluid. This liquid is pumped to tanks on the roof, from where it will trickle through the containers of plants, below the soil and out of human contact. All the tanks will be aerated and will be adaptations of the Infinity BioSolve reactor tanks. The hydroponics will finish off the cleaning, meaning that the effluent that drains off will be clear water. The rate of flow of fluid through the hydroponics system will be determined by gravity and a series of orifice plates that will control the feed pressure and volume. Theoretically the water run off can be collected and recycled. However, it is possible that after transpiration there will be little run off.

A third by-product of sewage treatment is methane gas. Where the technology exists; this can be collected and used as a fuel. For example, in October 2010, Didcot sewage treatment plant in Oxfordshire, UK, became the first plant in the UK to incorporate bio-methane with fuel gas that was being piped to customers’ homes. This biogas would otherwise be released as a greenhouse gas to the atmosphere.

Despite the reliance on advanced technologies of large scale methane capture, the principle has long been used in small scale biogas digesters where there is no municipal sewage treatment infrastructure available and fuel gas must be delivered in pressurised canisters by road or river. For example in rural Sarawak the cost of a cylinder of gas can be as much as 6 times the normal price in urban areas and this does not count the environmental costs (such as carbon emissions) of diesel fuelled transport [iii].

Biogas digesters are a viable low budget, low scale means of dealing with waste and fuel production in rural areas. Malaysian design consultant, Arkitrek have been experimenting with biogas digesters in Sabah. Their design is based on the Deenbandu model that has been successfully used in India for many years. These digesters are usually built by skilled bricklayers. However, Arkitrek wanted to test whether unskilled workers, including volunteer tourists, could build them. The design uses a brick tank with a ferro-cement gas holding dome. Gas is kept under pressure in the dome by water in a displacement tank. Waste is held in the tank for a minimum of 30 days where it is digested anaerobically. The resultant biogas (mostly methane) can be collected and used for cooking and lighting.

Biogas digesters are best fuelled with animal dung mixed with water (pig and poultry dung has the highest energy value per volume). They can also run on sewage and selected kitchen waste providing the quantity of water mixed with the waste is strictly controlled. If there is too much water (such as is used by a typical WC flush cistern) then the biogas digester will not operate properly. Grey water must therefore be treated separately, for example by using a conventional septic tank connected to a leech field or constructed wetland. The effluent sludge from the digester must be dried and composted, after which it can be used as fertiliser. It is thought, that if biogas digesters can supplement septic tanks then two major infrastructure considerations – human waste disposal and energy production – can be dealt with in the same place.

Two prototype digesters are under construction in Malaysia with the help of volunteer organisation, Camps International from the UK. One is at a longhouse near Kudat, Sabah and another is at a wildlife sanctuary in Sepilok, also in Sabah. The design must be robust to tolerate construction by unskilled labour and users must be educated to limit water put into the digester. If the digester is to be fuelled by raw sewage then squat pans or toilets need to be low flush volume or easy flush design and grey water (such as from floor washing) must not drain into squat pans. Both prototypes are currently being tested.

Once the construction and operational challenges have been overcome, the potential for rural areas to have an effective waste treatment and energy production system that is cheap and straightforward to construct is one that Arkitrek feels is worth pursuing. A high standard of environmental water quality is essential in our food producing regions as well as in urban areas.

Standards for effluent discharge set by local regulations in South East Asian countries are often not as high as those set by countries such as Europe and Japan. In other words; water resources are not equally regarded as precious and worthy of protection. Until the time when it is possible to bring standards to the same level, the implementation of environmentally responsible sewage treatments must be industry driven rather than government driven. This process can be helped by new products being brought to market, by clients making an investment in sustainable projects and by architects finding innovative solutions and integrating them with buildings and infrastructure design.

[i] Time; Oct 4, 2010; Walsh, Bryan; Code Blue. One woman’s quest to save our oceans the simplest way: protect them
[ii] Jeremy Hance. mongabay.com (September 29, 2010); Rivers Worldwide in peril: society treats symptoms, ignores causes
[iii] The Star; Dec 6th 2006. Gas subsidy for interior

More by Sarah Greenlees The Tower to the Memory of a Tree

Not so much as a fart of Biogas

You will recall from my earlier post that we have been building a biogas digester at Camp Tinangol with Camps International.

The aim of this prototype was to test whether a biogas digester could be built using unskilled labour, such as gap year students on a Camps International trip. We based our design on the well documented Deenbandhu model with a ferro-cement fixed dome for storing gas under water pressure.

Camps International are also working with us on a second digester at the Bornean Sun Bear Conservation Centre (BSBCC)

Our results to date have been inconclusive because the BSBCC digester is still under construction and at Tinangol, although complete, the digester has failed at the testing and commissioning stage.

Reasons for the failure at Tinangol are due perhaps not surprisingly to poor workmanship. The body of the digester was not watertight and the fixed dome was not air tight.

An important lesson that we learned is that it’s better to test a digester with water and a compressor first before you fill it with shit!

A week or so after filling the Tinangol digester with ‘fuel’ the camp residents reported a foul smell and grey water seeping up through the ground at the perimeter of the digester. We had to pump all the shit out and flush the digester twice with fresh water before it was clear to dig out around the digester back to slab level and pour an extra skin of concrete around the entire thing to block the multiple leaks.

With water-tightness rectified and tested satisfactorily we turned our attention to testing for air leakage. We did this by hiring a compressor (the type used by builders to power nail guns) and pushing an air hose down the displacement tank and into the body of the digester so that we could fill the dome with air.

Using a home made ‘U’ tube pressure gauge we sat back and waited to see whether the digester would hold the 10cm static head of water pressure that we had pumped in.

The pressure deflated faster than a whoopee cushion under a fat man’s arse.

With reference to a schoolboy trick for locating bicycle tyre punctures we coated the digester dome and gas pipes with a viscous washing-up-liquid solution and pressurised it again. By this method we quickly located gas leaks in the pipes that distribute biogas to the kitchen stove. These leaks were easy to fix with some extra plumbing tape and a silicon gun but the ferro-cement dome stubbornly refused to reveal the location of other leaks.

The digester still would not hold pressure.

We tried testing the dome at different pressure levels to see if we could isolate how far up the dome was the point of leakage. This made no difference except that at higher pressures the deflation curve was not uniform. There would be a fast initial deflation followed by a slower rate and then it would speed up again. Bubbles of air also appeared in the water in the displacement tank although we knew from the pressure gauge that the level of air in the dome was well above its underflow level at the displacement tank opening.

Our conclusion from these tests was that there were multiple points of leakage and that some leaks may be being transferred from one place to another through interstitial cavities in the ferro-cement dome and brickwork of the displacement tank.

We were not able to pinpoint the location of any of the leaks.

As the digester had already received multiple additional coatings of cement plaster we decided that it wasn’t worth putting on an extra layer to try to stop the invisible leaks.

Our strategy now is to wait until the BSBCC digester is finished and put that through the same tests. Being our second digester (more experience) and a little larger (easier working access) the quality of plastering work on this Mark II Prototype is a lot better than the first.

Meanwhile the effluent pipes in the Tinangol digester have been modified so that it can function as a normal septic tank.

If the BSBCC digester also proves to be incapable of storing not so much as a fart of methane we will look at an alternative biogas digester design; most likely with a floating gas holder where the use of prefabricated HDPE or fibreglass would more or less guarantee air-tightness.

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11 October, 2009

Poop 2, The Sequel.

By Rory
View original post on the Camps International Project Blog

Following on from my last post about the Bio Gas collector (Human waste is where its at), I want to now run you through our progress to date. Before I do that below is a sketch drawing of whats involved in the construction. Basically several tanks joined together, a bit of wizardry with water pressure and balancing and a domed lid with a pipe coming off it.

Our plan is to build our very own bio gas collector as a prototype so that we can demonstrate the technology and building requirements to interested parties in the hope that they can role it out in different locations. In addition we hope to be able to start building them for remote rural communities with sufficient “poop” capacity to make them worthwhile. We may start this process in the community of Tinangol where people live very closely together and it lends itself to their use – we will see!

The process of building the collector is actually relatively simple which I will show here with a number of pictures. Firstly a large hole is dug and a concave concrete base laid.

Then a basic double skinned brick wall is laid to just above the surface including the rectangular balancing tank on the side.

These are then skimmed with concrete to make sure they are water (poop!) tight.

Drains and sewage inlet/outlet pipes are then laid including an additional “mixing” inlet to allow kitchen and other biodegradable waste to be added.

Then comes the tricky part of constructing the dome – we have opted to do this using ferro-cement building technique – this is basically iron rods bent into shape, covered with a fine mesh and then plastered with cement to provide a smooth internal and external surface. This is then secured onto the main tank using more iron work ensuring it doesn’t fly off under huge gas pressure.

The final task is connecting all the piping and diverting the “poop” flow into the digester.

In order to fully monitor and assess its efficiency we have devised a simple volume measurement for waste going in – and people think I am mad when I tell them but we plan to have a clip board in the toilets. On this people will tick whether they “laid” a small, medium or large (I will have to quantify these!) “package” and also what was its consistency (fluid, normal or rock) and how many pans of water to flush it away. Hopefully this should give us an idea of volume of waste to water.

And that’s about it so far – we will be connecting the one at Tinangol very soon and will be building one for the new Sun Bear Conservation Centre at Sepilok Orangutan Sanctuary this month. I will keep you posted of progress and if you are really interested the volumes!! Happy crapping everyone!!

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