Plant Operation



There is a debate going on regarding the suitability or otherwise of package sewage plants as a viable option for the treatment of small to medium domestic outfall treatment.

This paper is to support our view that indeed our modular sewage plants are the preferred alternative, on condition that they are correctly designed, constructed and maintained.



While all sewage treatment protocols follow roughly the same pattern of sequential steps a package sewage plant can be defined as a system containing all the indiviual steps in one system allowing the flow to go through the treatment unaided.



Modular sewage plants clarify the incoming effluent using physical steps (settling and floating), bacteriological steps (anaerobic and aerobic digestions) as well as a chemical or physical step to sterilize the final effluent (chlorine, ozone, UV light or ACN etc).

The level of contamination is usually expressed as either the COD (chemical oxygen demand) or BOD (biological oxygen demand).  These values show how much oxygen will be needed to oxidize all the contaminants present; the higher the number the more contaminated the effluent.

Generally carbonaceous contaminations are remove by initial anaerobic treatment (producing volatile methane), followed by anaerobic step, resulting in the formation of carbon in an oxidized form, carbon dioxide, (also volatile).  Because of this, plants always run in the sequence anaerobic followed by aerobic to optimize the carbonaceous contamination removal.

Unfortunately, sewage effluent also contains nitrogenous contaminants.  For these, this process (anaerobic followed by aerobic) is not ideal.  Under the (normally) final aerobic stage the nitrogenous contaminants (amines, ammonia and nitrates) are oxidized to nitrates (non-volatile).  This is still a seroius contaminant.

This needs to be reduced (anaerobic) step in which nitrates are reduced to nitrogen gas (volatile).  The effluent therefore has to be re-cycled, partly at the very least, back to the anaerobic chamber.

In our modular plant five stages are always present, being initial removal of solids, followed by an anaerobic digestion phase, then aerobic digestion, final settling and then sterilization.


  • Composition differences

Domestic sewage from different sources differs.  These differences stem from a number of factors.  A  change house effluent will be very different from a kitchens; the diet of the users, (the effluent from a mine hostel eating area is totally different to the effluent from an up market restaurant); the socio-economic status of the population served; does the system exclude or include storm water; will there be sand in the effluent (e.g beach resorts); presence of non-domestic effluents like industrial effluents etc.

Degree of purification required

The treatment is also infuenced by the degree of purification required; much higher demands are placed on a system to generate an effluent suitable for discharge into a sensitive river than on one producing irrigation water for pasture land.

In our modular sewage plants optimum performances is achieved by adapting the size of the different chambers to create the optimum residence time in each treatment phase.  Additionally the degree of recycle also influences the design.  Our modular sewage plants are inherently adaptable to cater optimally for each set of conditions encountered.



The first step in any treatment process is the separation of gross solids that might be present in the effluent stream.  This is achieved by a screening step in which materials such as rags, plastics, sanitary pads, condoms etc. are physically removed.  This is essential as these solids will quickly block the system and reduce its capacity.

Following on this, the grits need to be removed.  These are sand particles, small stones etc. these materials are not amendable to bacterial breakdown in the system and should therefore be physically removed.



Following the inlet works the effluent flows through a series of chambers that allows it to separate into three : a material that floats materials that settle and the "middle cut" that has been cleaned of the both settlers and floaters.

This stage is run under anaerobic conditions (absence of air). While in this chambers bacteria will begin with the breaking down the macromolecular contaminants in the effluent.

The reaction taking place here can be simplified to :

Large organic molecules                 to             smaller molecules + methane (CH4) (gas)

                                        Anaerobic bacteria



The middle cut flows to the oxidation chamber where, in the presence of air (aerobic conditions) the breakdown continues.

The reaction taking place here can be simplified to :

Smaller molecules + O2                  to            CO2 (gas) + H2O

                                        Aerobic bacteria

As well as : Amines, ammonia and nitrates       to NO3 (nitrites)

                                        Nitrifying bacteria



The effluent from the oxidation chamber passes to the final settler.  By now practically all the faecal solids have been digested, the solids now present are mostly bacteria generated in the different stages.  This settles in the final stage.  It is a valuable fraction as it contains large numbers of active bacteria well adapted to the specific effluent.  This is then collected and returned back to the start of the process as RAS, return activated sludge.



The stage helps to :

  • Clarify the final discharge
  • More significantly, is the return of bacteria to assist in the breakdown of contaminants in the other chambers
  • Assists with the breakdown of nitrates returning them to the anaerobic part of the plant (sse paragraph 3 above)/

The reaction can be simplified to :

NO3                     to          N2 (nitrogen gas)

                 De-nitrifying bacteria



The final effluent is then sterilized to create a safe discharge, within the appropriate discharge specifications.  We offer a guarantee that the discharge is within specifications as requested.  This achieved by the use of the unique ACN disinfection product which is registered under the act 36 of 1947 registration number L4846.  This product is environmentally friendly and not harmful to humans, animals and plant or bird life.



Because the flow through the plant is by gravity, and self-controlling it runs with no high level management nor intensive operator inputs required.  At most it needs checking to see that the different electric components are running and that there is an adequate supply of re-agents (bacteria and sterilizing agent) available.



The department of Water Affairs has different specifications set out for the final discharge to be acceptable.  This is determined how and where the effluent is to be discharged.  Again the adaptability of our design allows for the required degree of purification to be met on a continual basis.



The main advantages of our package sewage plant are therefore:

  • High efficiency results in smaller and less expensive plants, saving on area as well as capital costs.
  • The mode of operation allows for underground placement, plants do therefore not spoil the view.  The area above the plant roof can be used as parking area, recreational area etc.
  • Maintenance is un-complicated; no high level controls are needed.
  • No off-odours are generated.
  • Plants are modular, can be enlarged as the need increases; phased CAPEX spending.


Typically these plants erected where:

  • The access to municipal lines are either non-existent
  • Prohibitively expensive due to distances
  • Municipal plants are already overloaded
  • These would include game lodges, holiday resorts, golf estates, mines and other rural applications, temporary facilities such as road working or dam building camps and arid areas where water reclaiming is essential etc.


1  =  Solids separation chamber

2  =  Primary anaerobic digestion chamber

3  =  Secondary anaerobic digestion chamber

4  =  Oxidation chamber

5  =  Fianl settling (RAS) chamber

6  =  Sterilisation chamber

7  =  Screen for solids discharge

8  =  Return activated sludge

9  =  Bacteria dosing

10 = Aeration

11 = Final water and sterilisation dosing


  • SAM-RAS Principle
  • Self-controlled gravity flow
  • Small footprint (forced aeration)
  • Underground
  • Low service required (few moving parts)
  • Low management required (simple process)
  • Almost odourless
  • Modular
  • Adaptable


The sewage plant has been constructed with a dividing wall separating the plant down the centre.  This construction allows for maintenance without disrupting the flow through te plant.  Once the maintenance is complete, the plant is returned to full production.
















Copyright  2013 Uthingo Environmental Services