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Septic Systems eWisa 'Read More'

Septic Systems: Rural Sewage Disposal

Critical Points:


To treat human bodily wastes satisfactorily, Septic Tanks must be;


1. Large enough for the load they receive.


2. Designed and Constructed properly


3. Not abused with chemicals, toxins [organic inhibitors] and solids.


History: Septic systems are used to treat sewage from [usually] single dwellings in districts where municipal waste water treatment works are not available. Larger systems are also used by rural industries to cater for the disposal of staff effluent They are thought to have originated in France during the early 1800s where they were developed to deal with human wastes generated in the new towns that were expanding rapidly to support the industrial revolution which followed the regime of Napoleon Bonaparte.

Until that time, the wealthy used buckets in an ablution room within their house, and the staff [chambermaids] emptied the contents when necessary into the same cesspit latrine adjoining the house that they used themselves. Some larger buildings, country houses, abbeys etc. had quite sophisticated sewers that drained from within the house to a cesspit and associated reed bed in the grounds. Faecal solids were flushed along the pipes [by household staff] using random buckets of wash water.

Large cities had networks of brick or stone built gravity sewers that drained all effluent to the river around which the town had developed.

Such disposal methods, along with the vermin that flourished upon the discarded refuse, gave rise to frequent outbreaks of illness, because rivers, such as the Thames and Seine, became contaminated with sewage, although they were used for moving bulk or heavy goods by boat in those days.

Septic Systems comprise:


1. Septic Tank with at least two chambers. Primary which receives and Secondary, the maturation chamber where the biological process continues


2. Soakaway / leachfield*



1. A septic tank has three main functions:


I. Separation of sewage solids from liquid; the faecal solids float to form a scum on the surface of the Primary chamber


II. Reduction of scum[stools & toilet tissue] in the Primary and dissolved COD [urine] by anaerobic bacteria; in both chambers


III. Storage of inorganic solids and minerals [septage] as a fine silt on the floor; to be removed by spade or suction periodically [every 10 - 25yrs]


Septic tanks work because:

Natural anaerobic bacteria and associated microorganisms living within the septic tank will adapt to degrade human sewage. To do this the tank must be:


Properly sized for the load it is receiving – number of people resident in the dwelling.


Constructed correctly;


1. Always with inlet and outlet ‘baffles’ [usually ‘T’ junctions on their sides],

2. At least two chambers:

a). Primary [solids digestor] and

b). Secondary [maturation]


Only receive black [sewage] water, not grey [wash] water**


The Soakaway [also known as a French drain/Drainagefield / Leachfield is an integral part of the system. It allows partially treated effluent from the Secondary of the septic tank to ‘soak away’ into the surrounding soil where it is progressively ‘polished’ back to potable quality as it seeps ever further downwards through the natural ground towards the underlying ‘water table,’ being progressively ‘purified’ by soil bacteria as it goes.


The efficiency of the soakaway depends upon:


1. Permeability: The porosity of the soil


2. Size: The length of the trench in relation to the number of residents in the dwelling


3. Construction: A loose covered, narrow trench with porous aggregate [media] on the outsides, all wrapped in a geo-textile blanked and [loose], it must be constructed along the contour, to a recognized design – a pit dug in the ground full of builder’s rubble is no good at all.


4. Level: It need not be straight, but it must follow the contour, without any gradient


*NB: Where soil conditions do not favour soakaways there are two alternatives:


a) A storage ‘conservancy tank’ which is routinely pumped out [usually] by the municipality.


b) A small ‘Package Plant’ which refines the effluent to discharge quality. Several septic tanks may be connected to a common sewer manifold served by one package plant. This is often referred to as a ‘small bore system’.


**NB: Older publications, pre circa 1990 advocate the inclusion of grey water in the septic tank. That was when tanks were larger and generally before the ‘off the shelf’ roto-moulded plastic tanks, commonly installed today, were freely available. Also, in those times, the excess N & P, found in laundry water especially, were seen as advantageous to the development of the faecal degrading bio-mass in the Primary. However, modern [post 1990] foodstuffs frequently contain high levels of refined micro-nutrients which will fulfill the same task in the smaller volume tanks that economy usually dictates the builder installs today. Many of the elements in modern food contain complex molecular structures which, emulsified in waste water, take time to break-down. Hence adequate ‘residence time’ is essential to allow this to happen; we recommend 18-21 days. Many elements resist degradation which is why high performance biological additives [such as our Bio-Systems STR] are becoming ever more important in balancing the equation.

How Septic Tanks Work

Septic Tanks.


a) Rely upon naturally occurring anaerobic microbes****, the activity of which is to degrade organic wastes [faeces & urine] into simpler, organic compounds: natural gasses, water and inorganic radicals.


b) They work without input of energy, other than that provided by gravity [water flow].




1. Septic tanks should have at least two separate chambers, separated by a common wall which is perforated two thirds below the surface of the liquid to allow effluent to move from the Primary chamber to the Secondary.


2. The inlet and outlet are at the same level in the ground and both fitted with side mounted ‘T’ junctions open at their two ends. [see drg]. This is important as it allows incoming wastes to enter the Primary chamber beneath the crust and prevents [stray in the case of overload] floating solids escaping from the Secondary to foul the soakaway, whilst at the same time, permitting rodding from the inspection cover frame above in the event of a blockage [nappies, toilet paper, etc].


3. The incoming flush of raw sewage displaces an equal quantity of semi treated effluent from Primary to the Secondary and in turn from the Secondary out to the soak-away/leachfield.


4. There are at least two inspection covers [manholes], one over each ‘T’ junction to facilitate rodding and cleaning. Larger [multi chambered tanks should have inspection covers, above the inlet and outlet [preferably fitted with their own ‘T’ junctions] to each chamber. These covers must be a gas tight fit and strong enough to withstand passing traffic [heavy duty cast iron lids are necessary in roadways]. The grooves in the frame must be free of soil and grass. Grease is usually applied to ensure an airtight seal.


5. Large tanks are multi-chambered, - each one having an inspection chamber accessing both inlet & outlet - so that the perforated cross walls will provide support for the lid.


6. Bio-Gasses generated by the microorganisms during the degradation of the sewage escape back up the inlet sewer and are vented at the dwelling [the stack pipe that just perforates the eave and allows gas to escape above the gutter – this also prevents the formation of a vacuum when the toilet chain is pulled  which would otherwise impede the passage of the ‘flush load’ down the sewer to the tank]. Large [multi-chamber] tanks should be vented above the waterline to allow this *3


7. The Primary tank should have an operating surface crust, 10 – 50mm thick floating on the surface.


8. The Secondary tank should resemble the sky on a dark, cold night; black water with a myriad tiny ‘stars’ – bubbles of bio-gas.


9. A healthy septic tank will not generate odour.


*3The gas could be captured and used to fire a stove or small electricity generator. In theory, a family of four could cook supper on the gas produced daily from their septic tank. We understand that this has been practiced by the rural Chinese for centuries. Enquiries on

*4Most non-pathogenic microbes will adapt exponentially to degrade the sewage substrate [food] they receive in the septic tank, but this takes time. However, to ‘kick-start’ the process and to ensure that a tank works efficiently from ‘day 1’, we advocate ‘seeding’ with STR which contains specialist sewage bacteria that will immediately go to work to degrade the raw sewage. As they are already ‘sewage trained,’ very little adaption time is necessary and the septic system benefits from the initial boost which leads to sustained biological activity manifested in the efficient break-down of sewage. Also, tanks which are temporarily overloaded, [e.g. when the family comes for Christmas], can be ‘boosted’ by the addition of Bio-Systems STR or W-A-W enabling the bacteria in the tank to meet the challenge

The Primary receiving tank holds sewage solids floating on flush-water & urine. Microbes sourced from the ‘starting’ bio-mass*5, commonly natural latent spores from the surrounding air, soil and attached to the internal tank surfaces during construction and a small %age from the human digestive tracts, passed in the initial faecal sewage.

It is here that the sewage solids are broken down into simpler substances.

The chamber should have a thin light brown scum [15 – 50mm] floating on the surface.

The Secondary is a maturation chamber where the partially treated effluent is further organically ‘cleaned’ by the microbes prior to discharge to the soakaway. There should not be any surface scum, and the water typically appears ‘black’ [hence the name ‘black water; referring to sewage].

The Dividing Wall retains sewage solids, and scum in the Primary chamber whilst permitting the solid-free, partially treated effluent to migrate to the Secondary for maturation.

The degradation in COD within the septic tank is commonly 4,500 – 5,000 ppm , depending upon Residence time [the longer it stays, the cleaner the microbes make it].

Typically, incoming raw sewage, [black water only] can have a COD value of 5,500 -6,000ppm. [based on the average production per person of 100g COD /day]. Out flowing effluent, from the tank, to the soakaway could be expected to be 800 -1,100 after 18 days in a well balanced system. So a septic tank does work – very well!

The faster the flow through the tank, the less the residence time, so the less the break-down of the raw sewage and, therefore, the greater the load on the soakaway….leading to trouble!

How Septic Tanks are Constructed.

Positioning on the property

Frequently local bye-laws dictate where septic systems may / may not be positioned. Refer to your Title Deeds or local authority before construction commences.

If ground frost occurs, ensure the tank will be below the ‘perma-frost’ level

Conventionally septic tanks are an oblong box built with 230mm brick walls on a 150mm r/f concrete in-ground slab. Plastered and [preferably] epoxy painted on the inside, they must be water tight and not leak. The lid can be made with a 25mm grano screed on plastic on abutting pre-cast concrete lintels. Care must be taken to ensure loading strength to withstand passing traffic. The two [minimum] chambers are separated by a common, perforated transverse wall. Always try to size the tank to standard available building materials. The max depth of the ‘hole’ is generally 1.9m. The depth of the occupying effluent [top of concrete floor to surface of liquid contents is +- 1.5m. Any deeper, they would have to use a back-hoe and shutter the sides to prevent collapse; both expensive! This is worth bearing in mind when calculating the volume of a septic tank. Take the known length x known breadth [450mm less than the overall dims of the roof slab, and multiply by 1.5. This gives you cubic metres [m3]. Multiply by 1,000 to give it in litres

The effluent chambers are conventionally around 1.8m deep overall with an effluent operating depth of 1.5m. This allows +-300mm free board between the effluent surface and the underside of the lid. The overall hole in which the box is constructed is usually 1.9 - 2.2m deep: Any deeper is impractical in most soils, without the walls collapsing.

The box has a dividing wall, perforated [3 x missing ½ bricks], or twin ‘T’junctions in larger tanks – with removable inspection covers over], approx 1.1m below the effluent surface and +- 200mm -500mm above the floor.

Plastic ‘torpedo’ roto-moulded tanks are also available, but due to the reduced surface area of the scum blanket, they are not as efficient as brick tanks, [because of their shape] and you may need around 40 -50% more capacity for the same volume handled by a brick tank, which usually makes them impractical – especially on larger installations above 4 residents per dwelling. They should be encased in r/f concrete to avoid movement in the soil which frequently causes distortion and the rupture of the drain-pipe junctions.

Septic Tank Sizes

This is easy to calculate on the basis of flushes per person per day. It equates to approximately 60lt per person, [7 toilet flushes per person of 8lt each flush; this being the discharge capacity of an average toilet cistern] which, taking the recommended ‘residence’ time of 21 days into account, indicates a minimum tank size of 1,200lt per person. So for a family of five, and to ensure a 21 day ‘residence time’, you need a tank capacity of 6,000lt minimum. Remember also the friends that call, especially over weekends. If in doubt, rather go for a 7,500lt unit to be safe. The construction cost is only fractionally more.

NB: Surveys indicate that the average 5 person family will use 180 -220lt water per person per day. So that’s +- 1,000lt daily total [and includes bathing, showering, laundry, cooking, washing-up &c &c] so they would need a septic tank of 21,000lt to be safe !

But they would only use an average of 60lt / person / day for toilet flush. So that’s 300lt / day = 6,500lt / month, so a normal septic tank of 6,000lt capacity would be in order. The other 14,500lt of waste ‘grey’ water should go directly, via a grease trap, to the soakaway, or, better still, via a grey water treatment system so it can be re-used in the garden for irrigating the plants.

Starting a septic tank


*5. Traditionally, many rural dwellers believe that a dead cat or a sheep’s head will ‘start’ a septic tank. This is a fallacy. The bacteria within animal body parts are not the same as those found in sewage, but overtime – several weeks - , they will adapt to their new environment, and become a sewage digesting bio-mass.


As with municipal works, ‘seeding’ with sludge from a nearby system is also not satisfactory as the donor source will usually be suffering from a weak bio-mass.


Often people say that their tank is ‘working’ because they can see maggots or worms on the surface of the scum. The presence of these creatures has nothing to do with the sewage treatment process, other than to indicate that the tank is in a poor condition because the scum blanket is aging. Bacteria that digest the blanket from the underside are microscopic and cannot be seen with the unaided eye.


The correct procedure is to inoculate the new tank, when half full with Bio-Systems STR [or SK1 in cool climates] which will introduce the correct sewage degrading bacteria into the system. These will thrive and multiply many thousand fold and this ensures that the tank and drain fields start and run efficiently.

There is are advantages to the builder in constructing the septic tank at an early stage,

1. Saving on excavation as the site clearing machine that cuts the footings can excavate for the tank & soakaway at the same time


2. Negating the need for site toilet hire,


3. Providing the client with an operating system at hand-over.


4. The soakaway ‘media’ can be ‘added to’ with clean broken brick rubble and scrap stone as the building progresses, saving on site storage and cartage




1. If a septic tank is undersized for the hydraulic load [volume of effluent] it receives, then sewage solids are liable to be washed over into the soakaway which becomes blocked and the tank overflows.


2. ‘Pumping out’ the tank only gives a few days’ respite; because once blocked, the soakaway is very difficult to clean. [We have a proprietary product, SAC (Soak Away Cleaner) to achieve this task – but, being biological, it takes time to work].


3. Repeated ‘pumping’ – necessary to prevent frequent overflows and the surrounding garden from being soaked in raw sewage – is expensive and not the answer. It results in a depletion in the essential microbial ‘biomass’ living in the liquid in the tank, leading to sterility and complete failure of the entire system coupled with appalling odour from the rotting fats that are still in the tank and soakaway.


4. Very few ‘vacuum tanks’ are fitted with spate pumps necessary to extract fat sludge. These units are expensive [around R1,400 per hour yard - to – yard


Soakaway / Drain Field

A soakaway (also known as a ‘leach field’, or ‘French drain’) receives partially-treated effluent from a septic tank or grey water drain. Their purpose is to complete the job initiated in the septic tank. i.e. to clean the dirty water, by organically breaking down the suspended wastes and impurities, through the surrounding soil to a degree that makes it suitable for discharge into the underlying water table, from whence it will be collected in boreholes [or artesian wells] and piped to surface for re-use by others. The effluent from an efficient septic tank [correctly sized for its load – see above -], entering the soakaway will typically have a COD value of 800 – 1,100 ppm. When that same water joins the water table, the COD should be down to under 5 ppm, rendering it potable [fit for humans to drink]. Percolation takes several weeks, and depends upon the nature of the soil.

Typically soakaways contain a hard, insoluble media such as ‘clean’ crushed stone, clay brick ends or old car tyres. This hard substance acts as a host for a growing bacterial biofilm [slime] that progressively degrades COD (organic impurities) as the effluent passes slowly over and around it and then seeps out to the surrounding soil, where further natural soil microorganisms, attracted by the ‘food’ and self-adapted to the fractionally higher organic load contained in the nearly cleaned water, polish the liquid to potability [ultimately you can drink it]. Once in the earth the purification process is gradually continued by bacteria in the soil. The exponentially cleaned water then rejoins the underlying water table. Consequently, the design and position of the soakaway in relation to the slope of the natural ground and the porosity of the soil as well as the proximity of the underlying water table is essential to the efficacy of the system – and public health. Design Parameters which must be observed:


1. Porosity of the soil – soakaways won’t work;


a. In clay soils


b. In rocky terain


c. Where the natural water table is ‘high’ such as near or in wetlands


d. Where underlying bed rock/ transverse shale strata prevents seepage


2. Size of the septic tank – if it is to be a combined installation


3. Number of people living in the residence


4. Nature of the waste water


5. Slope of the ground


6. Proximity to natural watercourse


7. Distance from property boundary


A soakaway that works properly is designed to cater for the hydraulic load it receives and is a proper (easy-to-build) structure. Essentially, it should be a long, narrow, trench built on a rammed earth floor.

It must be dead level with walls of dry-packed ‘butterfly’ bricks, [or blocks on their sides] and with a layer of clean crushed aggregate sandwiched between two wraps of geo-textile – the outer one preventing the ingress of soil/sand which would otherwise sift in, blocking the interstices [gaps in the media]; the inner to prevent small particles of media aggregate falling into the central cavity of the trench – between the dry walls. The whole trench should be covered with loose, pre-cast paving slabs [to prevent accidental injuries and ingress of rubbish] and for ease of access.

How a Soakaway Works

Effluent enters the soakaway trench from the septic tank (see above). The water stagnates allowing bacteria washed over from the septic tank to continue the maturation process in association with those latent in the soakaway. Simultaneously, as the water ‘seeps out through the geotextile, organic wastes suspended in the effluent are snared by the mucilaginous slimes that have developed on the surfaces of the ‘media’ [the clean stone/brick ends/tyres] and are progressively degraded as they flow very slowly from the channel propelled by the incoming effluent and the pull of gravity towards the surrounding soil Meanwhile partially-cleaned molecules of water slowly soak into the interstices between the soil particles where further bacterial slimes thrive under the damp conditions – the microorganisms gaining their ‘food’ from the remaining COD in the semi-treated effluent.


The fluid, under pressure from that flowing into the soakaway, slowly ‘seeps’ [we use the word ‘permeates’] further and further into the surrounding soil following the gravitational gradient (minimal though it may be), and is progressively purified by the latent bacteria within the soil. Ultimately, sparkling clean, clear, potable water joins the underlying water table. It follows that the actual porosity of the soil, clay content, particle size and gradient are prime factors in the speed of bioaugmentation and the ultimate final discharge quality of the treated liquid sewage effluent. It also follows that if too many soakaways are present in poorly performing soils, toxic problems [overload] will result. This is especially the case with developing populations in cluster housing and if artesian wells (boreholes) are sunk to tap the underlying water table.


Where soil conditions are not conducive to soakaways, or where congestion occurs / is likely to occur, the local authority may decline permission to install soakaways and the septic tank will then have to discharge to a ‘conservancy’ tank the contents of which must be pumped out [most authorities have vacuum tankers serving this purpose] at regular intervals; for a fee. An alternative may be to install a small ‘package plant’*6 which does the same job as a soakaway, but costs more to install and operate; However, the advantage is that the treated effluent is usually re-usable for garden, agricultural, and with extra refinement, maybe potable purposes. [as in Windhoek and parts of London]


*6 Package Plants: see plants


Grey water considerations

Taken that the average person uses a total of between 120-180 litres of water per day, our family of five will use around 15,750 litres in the same 21-day period.

If this all goes to the septic tank, the total hydraulic load is approx 750lt / day. Divide into the 6,000lt tank capacity = 8 days’ residence time.

For the conventional roto-moulded, off the shelf, ‘large’ septic tank of 2,500lt capacity, the factor is only 2.4 days’

No septic tank will work this way.


Therefore, grey water must by-pass the tank to (preferably) its own soakaway; or it can be treated for re-use (see Re-use of grey water).

Ignorance/denial of this is why so many modern septic systems fail.

Maintaining your septic system


A. Ensure that grey water does not flow to the septic tank, which it must by-pass en route to the soakaway / drain field.


Care is needed to prevent hydraulic and organic overload conditions that will otherwise upset the smooth running of the septic system.

The septic tank

Toilet tissue: Use good quality twin or triple ply, soft tissue which absorbs water and disintegrates easily when wet and, even when used to excess, should not cause a problem in a tank of sufficient capacity. Agricultural and building staff sometimes use coarse paper or rags, and by rural legend, even mealie cobs!

No foreign materials: Only human sewage and toilet tissue should be flushed. Cigarette butts, ladies textiles, rubber goods should never be put in the toilet bowl. Place in the bin provided.


1. Chemicals: Proprietary ‘loo cleaners’ available in supermarkets are usually ‘safe’ in septic systems where Bio-Systems’ products are used, but avoid bleach and strong acid or alkali drain cleaners and chemicals.


2. Discolouration is usually caused by minerals in the flushing water, which in some rural districts, may not be to urban standards.


3. Additives: Use Bio-Systems STR (info sheet available at at least once a year. This will boost the microbial bio-mass keep the system healthy and prevent expensive pump outs.


4. Enzymes: Tend to upset the biological balance, they may appear to work for a while, but the system will soon suffer and give trouble. [ask]. Enzymes are proteins which act once and have to be replaced continuously. Most industrial enzymes are not suited to sewage treatment. Bacteria, on the other hand, produce the enzymes they need to degrade sewage.


The soakaway


1. Should run smoothly if:


a) It is sized and constructed correctly


b) The tank is respected and not subjected to overloads.


2. If the seepage reduces – usually due to overloading with trans-fats – and the system ‘backs up’ causing overflows from the secondary (maturation) chamber of the septic tank, then apply our Bio-Systems SoakAwayCleaner (info sheet available at This product will gradually degrade the fats that are sealing up the minute seepage holes, allowing the system to return to normal.


3. Check for pipe collapse or penetration by roots as these factors can be the cause of problems.


Common septic system problems

Septic tank systems are like cars, they need periodic maintenance to keep them performing at their peak.

Septic tanks Trouble is caused usually by one or all of the following:


1. Size: The system is too small for the job it is intended to do.


2. Fats - an overload of fats cause failure in the soakaway/leach field.


3. Death - the tank ‘dies’ because it has received a dose of toxic shock from harsh chemicals, bleach, acid drain cleaners or HTH.


4. Overload - too many people using the loo and/or too often.


5. Feast and famine - heavy usage over a holiday period followed by long periods of inactivity (commonly experienced at weekend hideaways).


6. Wash out – when grey water (from the bath, laundry etc.) or storm water (from entry drain points that aren’t ‘bunded’ properly) enters the tank and causes excessive hydraulic load, preventing the degradation of faecal wastes.


7. Overflow – when the water from the secondary chamber cannot escape to the soakaway/leachfield because the latter is gummed up with congealing fats, thereby preventing liquid ‘seepage’ to the surrounding soil.


8. Pump outs – also known as ‘vacuuming’ to remove excess water (usually caused by a malfunctioning soakaway). This is bad practice because it removes the aqueous environment containing the beneficial bacteria. The fats – the cause of the problem – remain stuck to the sides, on the floor and in the soakaway. Pumped out frequently, the bacterial count in the tank is increasingly depleted, and devoid of bacteria, it ‘dies’ and the problem worsens. Very few suction tankers are equipped with ‘spate’ pumps and high-pressure jets that ‘slurry-up’ and remove fats. Those rigs are very expensive [charged out at around R1,550 / hr yard to yard].



2. Soakaways


Soakaway trouble is usually caused by one or all of the following:


3. Size and shape of the installation: Designed is inadequate.


4. Hydraulic overload: Soakaway too small for the load.


5. Organic overload: Fats and COD washed from tank before degradation is complete.


6. Sealing: Fat from food wastes blocking the seepage holes in the final medium (soil).


7. Surface flooding: Because fats are sealing the seepage interstices in the soil.


8. Odour: Caused by rotting fat and (anaerobic) COD.


The BIO-SYSTEMS solution

First of all, let’s start off on the right foot and get the size and design correct! To help you, we offer drawings of both septic tanks and soakaway / drain fields in email format:

Bio-Systems SA have developed a tough blend of hardened microorganisms that degrade the complex components of modern sewage, particularly the complex, refined fats, oils and greases (FOG) that would otherwise congest septic tanks and cloy the walls of soakaways, sealing the essential seepage ducts between soil particles.


1. Bio-Systems STR - all septic tanks


2. Bio-Systems SoakAway Cleaner (SAC) – all drain fields


3. Bio-Systems Wunce-a-Week (WAW) and Weekender –a sewage conditioner intended for the weekend cottage


4. Bio-Systems DF60 – commercial kitchens


5. Bio-Systems Dop-a-Day - domestic users NB: If any one requires drawings of either septic tanks or soak-aways, we can provide them, for a nominal fee, sized to the volumes that they are anticipated to receive.


However, we recommend that you obtain a porosity assessment of your soil from a qualified soils engineer.

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