Ste Anne Island, Seychelles where we have installed two 100 m3/d Reverse Osmosis Plants to provide drinking water to a new hotel and resort
Description of Reverse Osmosis
Making sure you get a good Reverse Osmosis Plant
About HOH Water Technology
Description of Sewage Treatment
Making sure you get a good Sewage Treatment Plant
About the BMS Blivet


This section of the website provides a description of the sewage treatment process, and the different types of Sewage Treatment Plants (STPs) available. The focus of the discussion here is on “small” Sewage Treatment Plants serving populations from about 50 to 2,500. However many of the principles discussed also apply to large municipal or industrial plants.

Sewage Characteristics
The Sewage Treatment Process
Types of Sewage Treatment Plants
Operation and Maintenance Cost
Frequently Asked Questions

Sewage Characteristics

The goal of sewage treatment is to process sewage to the point that it will meet the Effluent Discharge Standards of the local authority, or be suitable for re-use, such as use for irrigation. To begin the discussion of the Sewage Treatment Process, it is necessary to know the composition of the sewage. Typical characteristics for domestic sewage are shown in the table below. A brief description of the terms, such as “BOD” and “TSS” is given at the end of this section, in the Glossary.

Sewage Characteristics
Constituent Typical Range, mg/l
Total Suspended Solids (TSS) 200 - 300
Biochemical Oxygen Demand (BOD) 200 - 300
Chemical Oxygen Demand (COD) 450 - 700
Nitrogen (total as N) 30 - 50
Phosphorus (total as P) 6 - 10
Fats, Oil, Grease (FOG) 50 - 150

Sewage from hotels and resorts is often significantly stronger than domestic sewage. Typical BOD values for hotels and resorts range from about 350 mg/l to 450 mg/l. Sewage from industrial and commercial establishments may also be much different in both composition and strength and should be evaluated on a case-by-case basis.


The Sewage Treatment Process

Pretreatment. Pretreatment refers to all the treatment processes done prior to Primary Treatment. Some pretreatment is done right at the sewage source, prior to collection in the sewer system. This includes grease traps at restaurants, oil separators at mechanical shops, lint traps for commercial laundries, and various other types of industrial pretreatment depending on the type of industry.

Pretreatment may also take place at the Sewage Treatment Plant (STP) itself. Depending on the characteristics of the sewage, pretreatment processes at the STP may include grease removal using a grease trap, and screening using a bar screen to remove larger solids such as rags. If there is a lot of sand or grit in the sewage, a grit chamber may also be installed.

Typical Sewage Treatment Process                                                            
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Primary Treatment. A schematic diagram of a typical sewage treatment process is shown in the figure above. After the sewage has been pretreated, it flows into a large tank, called a Primary Clarifier or settling tank, where Primary Treatment takes place. Primary Treatment consists simply of providing a tank with quiet conditions so that any solids in the water are given a chance to settle, and any grease or oil present can float. The size of the tank required to achieve primary treatment can be reduced substantially by installing plates, called lamella plates, in the tank which help to create the necessary quiet conditions.

For smaller STP’s, the tank where the primary treatment takes place usually also serves as the place where the sludge is stored. A common type of Primary Clarifier for small STP’s, for example, is a septic tank.

Primary Treatment reduces the Total Suspended Solids (TSS) in the water by about 50 to 75% and reduces the Biochemical Oxygen Demand (BOD) by about 25 to 35%.

Secondary Treatment. The readily removable solids are removed in the Primary Treatment process, but lighter solids and dissolved substances remain in the sewage. These are removed in the Secondary Treatment process, in which bacteria are grown which consume these substances. The resulting solids (bacteria) are then removed in a Final Clarifier.

The growth of the bacteria takes place in a tank, and they may be grown either attached to a surface, called a “fixed film” process, or grown in suspension, called an “activated sludge” process. The bacteria which accomplish the sewage treatment require oxygen to be healthy. There are many types of aeration systems for providing air to the bacteria, including blowers, surface aerators, paddle wheels, and pumps with air injection. For most fixed film treatment plants, the media on which the bacteria grow is first submersed in the sewage, and then brought out into the air, which provides air to the bacteria. In other words, the bacteria is brought to the air, not vice versa.

As the bacteria consume the pollutants in the sewage, they grow, and the excess bacteria flow into the Final Clarifier, also called a final settling tank or humus tank. As with the Primary Clarifier, this consists of a tank providing quiet conditions where the bacteria settle. There is usually a weir at the top of the clarifier, where the clear effluent passes over, and out of the STP.

For most small treatment plants, the settled bacteria mass is pumped back to the Primary Clarifier, as shown in the schematic diagram above, where the bacteria settles and is stored along with the primary solids.

Tertiary Treatment. For many sewage treatment applications, the process described above will be sufficient. However additional treatment may be required either to meet the requirements of the local authority, or to treat the water for another purpose such as irrigation.

In some municipalities, for example, if the effluent will be discharged into the ocean, the local authority may require the removal of the nutrients nitrogen and phosphorus to low concentrations. This is because nitrogen and phosphorus could promote unwanted algae growth in the receiving water. The removal of nitrogen is accomplished by bacteria in a two step process. In the first step, called nitrification, organic nitrogen and ammonia present in the sewage are converted to nitrate. This is followed by the second step, called denitrification, in which bacteria convert nitrate to nitrogen gas, under low oxygen conditions. The nitrogen gas is discharged to the atmosphere. Phosphorus removal can also be accomplished by bacteria, but for small plants it is normally removed by precipitating it with a chemical solution containing an iron or aluminum salt.

The addition of nutrient removal processes to an STP will increase the cost of the STP substantially, usually by at least a third, and the operation and maintenance requirements are also increased.

If the effluent will be used for irrigation, the degree of further treatment required will depend on the type of irrigation system, what it is that is being irrigated, and the requirements of the local authority. In order of increasing treatment, the degree of additional treatment required might be: no further treatment, filtration only, or chemical precipitation followed by settling and filtration.

Disinfection. At most small treatment plants, disinfection is accomplished by chlorination, either using chlorine tablets or using a small mixing tank in which dry granular Calcium Hypochlorite is dissolved and then dosed into the effluent using a metering pump. To be effective, there should be a small chlorine contact tank downstream from the point where the chlorine is added, to give the chlorine time to disinfect, before the effluent is discharged.

Ultraviolet (UV) light may also be used for disinfection. It has the advantage of leaving no residual in the effluent, which is beneficial when the effluent will be discharged into a sensitive environment. Disinfection with UV light has a higher capital cost and higher operation and maintenance requirements than disinfection using chlorine.

Sludge Removal and Disposal. Sludge removal and disposal is a key consideration for all STP’s and it is especially important for those in remote areas. The frequency of removal varies for the different types of plants. For activated sludge plants, sludge removal is done every day to every few days, and so the plants are often built with a supplementary sludge storage tank or sludge drying beds, where the sludge removed from the plant itself is stored or dried for later removal. Fixed film plants usually have sludge storage incorporated in the design, and the frequency of removal ranges from one month to a year or more.

Sludge removal is often accomplished with a vacuum tanker truck, but if none is available, a sludge pump, or gravity can be used. Sludge disposal depends on the local facilities available. If there is a dump site nearby, this is a common disposal method. Other methods include application to land, drying followed by application to land, composting followed by application to land, or drying followed by disposal at a dump site. Sludge, in either liquid or dry form, is an excellent soil amendment.


Types of Sewage Treatment Plants

For a population of less than about 2,500, a fixed-film type of wastewater treatment plant is the best and most economical type of plant. This would include either a Rotating Biological Contactor (RBC) or a BMS Blivet. "Fixed-film" refers to the fact that the bacteria, which do the wastewater treatment, grow attached on a surface. Activated Sludge type plants, in which the bacteria grow unattached in a suspension, are usually not competitive unless the population served is at least 2,500.

Other advantages of a fixed-film type of plant, compared to activated sludge, is that they are simpler to operate, quieter, and consume less energy. They are also often modular, so that if a development project is done in two or more stages, it is possible to purchase just the amount of wastewater treatment plant required for each stage.

These types of plants will typically produce an effluent of 20 to 30 mg/l Biochemical Oxygen Demand (BOD) and 30 mg/l Total Suspended Solids (TSS), which will meet most discharge standards, and can also be designed to meet other standards if necessary. All have proven track records of reliability when designed and operated properly.

As suggested, activated sludge plants become cost effective for larger populations. However, if the larger population is located in separate areas, it may be less expensive to provide localized treatment with a fixed film-type plant in each of the areas, rather than collect and pump the sewage to a larger centralized plant, since the expense of the sewage conveyance system is avoided.

BMS Blivets. BMS Blivets were developed in the late 1980’s and the first commercial plant was put in operation in 1992. The principle of operation is essentially the same as for all wastewater treatment plants providing a secondary level of treatment. That is it includes a Primary Clarifier and sludge storage, followed by biological treatment with bacteria, followed by a Final Clarifier. Where the Blivets differ, is that they provide all these processes in a single prefabricated plant.

Typical Blivet installations are shown in the following photographs. These Blivets serve towns in Ireland which include residences, restaurants, pubs, and other commercial establishments. The first photo shown is actually two Blivets installed in series, with the second smaller Blivet in the background used for nitrification, which is required by the local authority. The larger plants shown in the photos below (the ones with five hinged covers) each have the equivalent treatment capability of the RBC shown above.

BMS Blivet Installation (Buried)                                                                 
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BMS Blivet Installation (Above-ground)                                                      
click to enlarge

Under the hinged cover on the right side in the photos is the primary clarifier. Sewage entering the primary clarifier flows up through lamella plate settlers. Solids settling out here are retained and stored in the large area under the biological treatment portion of the plant.

Following primary treatment, the sewage then flows into the Aerotor; the name for the biological treatment section of the Blivet, which is shown in the photograph below. The bacteria grow on a high-surface-area media inside the drums. The drums are mounted on a shaft which rotates the drums. An Archimedes screw inside the drums aerates and pumps the water through the drum as it is rotated

View of Blivets with Hinged Covers Open                                                    
click to enlarge

Water enters the first drum, where it is treated, then in turn enters the next drum and so forth. As the bacteria grow on the media and the bacteria layer becomes thicker, they start to slough off and are carried off with the effluent passing through the drum. The effluent/bacteria mixture then enters the final clarifier, shown in blue on the drawing, and at the last hinged cover at the left of the photos, where the bacteria settle. Finally, the clarified effluent flows over a weir and out of the plant. The bacteria which settle out are pumped back to the Primary Clarifier where they settle and are stored in the sludge storage area, along with the primary solids.

For a BMS Blivet, any of the methods described above for disinfection of the effluent, and sludge removal and disposal may be used.

Rotating Biological Contactors. Rotating Biological Contactor’s (RBC’s), also called bio-disks, have been in use for approximately thirty years. Referring to the schematic diagram of the sewage treatment process above, an RBC system usually uses a septic tank for the Primary Treatment process and sludge storage.

The bacterial growth tank consists of a series of disks attached onto a shaft which is slowly rotated. The disks are large and placed close together to give a large surface area for the bacteria to grow on. As the disk rotates it dips into the sewage, and then back out into the air, which both exposes the bacteria to the sewage, and provides air for the bacteria.

A photograph of a typical RBC installation is shown below. This is an RBC which serves an 80 room hotel. The septic tank is behind the fence in the back, and the three large cylindrical objects in the middle are the rotating disks.

Typical RBC Installation                                                                           

As the bacteria grows on the disks, the bacterial layer becomes thicker and thicker until it breaks off and flows into the final clarifier. In the final clarifier, the round tank shown in the foreground of this photograph, the bacteria settle and are pumped periodically back to the septic tank behind the fence. At this particular plant, there is also an option to pump the sludge from the final clarifier to a sludge drying bed.

For an RBC plant, any of the methods described above for disinfection of the effluent, and for sludge removal and disposal may be used. At the plant shown, the clear effluent from the final clarifier then flows into the chlorine contact chamber, the concrete tank at the right background of the photo, where chlorine is added using chlorine tablets. At this plant, the effluent from the chlorine contact tank flows to a soakaway pit. When the septic tank accumulates enough solids, it is de-sludged using a vacuum tanker truck or submersible sludge pump pumping to a tanker truck.

Comparison of the RBC and BMS Blivet STP Systems. A comparison of the RBC and Blivet STP Systems is given in the “About the BMS Blivet” section, which can be opened by clicking the link at the left.


Operation and Maintenance Costs

O&M costs for both the RBC and Blivet systems are quite low, which is one of the reasons “fixed film” systems are popular. Electricity costs are low as both types of systems have only two motors: the one that rotates the disks (or drums) and the sludge return pump. Both motors are usually 1 kW (1.33 HP) or less.

Labor costs are also minimal. The labor requirements for these types of systems are typically 100 to 150 hours per year, and they are not complex systems so that it isn’t necessary to hire a highly skilled technician for the work. It is, however, important that the operator be carefully trained by the STP manufacturer.

Similarly, chemical requirements are low, consisting only of chlorine, usually in the form of Calcium hypochlorite in granular or tablet form. Other than the chlorine and lubricants for the bearings and gearbox, there are no consumables. Spare parts usually consist of shaft bearings, couplings, and perhaps a spare motor and sludge pump depending on the remoteness of the location.

The cost for sludge disposal will depend on how it is accomplished at a particular STP, and should not be overlooked.

The STP manufacturer should provide the customer with detailed O&M costs taking into consideration the size of the plant and local costs.


Frequently Asked Questions

Some STP’s are rated by “Population Equivalent”. What is this and how does it compare to simply “Population”?
Population Equivalent (PE) is a term used for estimating the population that will be served by a sewage collection and treatment system. In many cases the estimation is simply the number of people served. For example if a house with four people is served, then the PE is 4. But for other cases, such as hotels, commercial enterprises, or industries, it is more difficult to establish the number of people served. So instead, a population equivalent is estimated from standard tables, or based on the experience of the estimator. The design of the sewage collection and treatment system, is then based on the equivalent population.

Can an STP be guaranteed to be odor free?
It is not possible to guarantee no odors for a sewage plant for two reasons. First, during sludge removal from the plant, there will be some odors, and second it is a biological process, so it can be upset. As discussed under ”Making Sure you Get a Good Sewage Treatment Plant”, another possible source of odors is that sewage in the collection system may become septic if it is not designed properly, which can result in odors being released at the STP and corrosion of concrete and metal components at the STP.

Having said that, odors can be just about be completely eliminated through proper design and operation of the STP and collection system. For extremely sensitive areas, odor scrubbers can also be incorporated.

In Sewage Effluent Discharge Standards, a given parameter often has a qualifying statement after it, such as ”as N”, or ”-N”. What is the purpose of these?
The reason for this is that constituents in sewage, such as nitrogen and phosphorous for example, can exist in many forms, and the forms themselves change throughout the treatment process (See “Nitrogen” and “Phosphorous” in the Glossary section). It is therefore convenient to express the concentration of a given constituent, say nitrate, “as N” or “–N”, which both mean “as nitrogen”. In some cases, the constituent may be specified as being “as Nitrate” or “-NO3”. This is done for clarity, because if it isn’t, people in the wastewater business will assume it is expressed “as N”.

This seemingly minor point actually has important implications for effluent discharge standards. For example, the Seychelles Effluent Discharge Standards have a nitrate standard of 15 mg/l, and it is expressed “as NO3-“, the chemical formula for nitrate. The value of 15 mg/l expressed “as nitrate” is the same as 3.4 mg/l expressed “as N”. This is a strict nitrate discharge standard. Someone not paying attention to the way the nitrate is expressed would assume the value of 15 mg/l was expressed “as N”, which is not nearly as strict.



Aerobic. An aerobic system or process, is one in which oxygen is present. If oxygen is not present, the system or process is anaerobic, or septic.

BOD. "BOD" or "BOD5 @ 20oC" stands for Biochemical Oxygen Demand and is one of the most commonly used measures of the strength of the sewage. It is a measure of the amount of oxygen which will be consumed by bacteria in oxidizing (stabilizing) the sewage. BOD is determined by incubating the sewage at 20oC for a period of five days and measuring the dissolved oxygen content at the beginning and end of the period.

COD. "COD" stands for Chemical Oxygen Demand, and is a commonly used measure of the sewage strength. It is a measure of the amount of oxygen required to chemically oxidize the sewage. For a given sewage source, the COD value will almost always be higher than the BOD value. One of the reasons for the popularity of COD testing as a measure of sewage strength is that a result can be obtained within a few hours, instead of the five days required for BOD testing. Also, for a given sewage source, the ratio of the COD to BOD, is relatively constant, so that the BOD can be estimated from the COD results.

Industrial Pretreatment. This refers to wastewater treatment that takes place at an industry, prior to discharge to a municipal sewage system.

Nitrogen. Nitrogen can exist in several forms in sewage. In raw sewage it is usually in the form of organic compounds containing nitrogen, such as proteins, or as ammonia. Following treatment, it will usually be present as ammonia, nitrate, or nitrite.

Some effluent standards require conversion of much or all of the ammonia to nitrate, as part of the treatment process, because the ammonia will exert an oxygen demand on the receiving water, and could cause oxygen depletion. This conversion is called "nitrification" and any of the secondary treatment plants described above can be designed to achieve this.

Some effluent standards will also establish a limit on the nitrate, ammonia, or total nitrogen because these serve as nutrients, and could promote unwanted algae growth in the receiving water.

Phosphorous. As with Nitrogen, Phosphorous can exist in many forms in sewage, and also in common, some effluent standards will establish a limit for phosphorous because it serves as a nutrient, and could promote unwanted algae growth in the receiving water.

Sludge. Sludge is a generic term which refers to the solids produced in the sewage treatment process, including the solids which settle out in the primary process, and the bacterial solids from the secondary process.

TSS. "TSS" or "SS" stand for Total Suspended Solids. As the name suggests, it is a measure of the solids content of the sewage.