Biological domestic wastewater treatment system Products plants





Biological domestic wastewater treatment systems (wwtp) FEL are installed in areas where connection to the central sewer system is impossible. The systems are intended to treat wastewater from residential houses and individual residential units. The treated wastewater after biological wwtp can be discharged to the environment without harming the ecology, i. e. treated wastewater can be released to the roadsides, melioration ditches or into the surface water bodies. In the absence of water body, treated wastewater can be filtered into ground through the filtration wells or fields. Also treated wastewater can be used for lawn watering.

  • CE marked according to EN 12566-3
  • 15 years warranty
  • High treatment efficiency
  • Easy to ship and install 
  • Minimum maintenance cost
  • Treated wastewater is clear and odorless     
Standard - is green gelcoat cover in the complekt (1). But we can offer and other : 
TELESCOPE (2) - The option is ideal for those who save space on their plot. Height-adjustable telescope + black (or green) plastic cover.
TRANSITION UNDER THE CARRIAGEWAY (3) - The choice is suitable if you plan to install the cleaning device under the carriageway, where a cover is provided with pads or the like. After using the transition, all you have to do is take care of the cast iron cover, which you can choose according to your needs. 
For more info about this instalation bellow, in the list "Instalation"    

More about the product



Before placing the wwtp on the vehicle, make sure that there are no sharp bulges which could damage the plant. In most cases plant is transported in a vertical position, conical part upwards and placing a inspection manhole on it. Also it is possible to transport the plant in a horizontal position. To avoid various damages during transportation plant should be securely fastened. For fastening use GRP, nylon or other non-metallic belts.

During storage the plant must be protected from damage and third parties, and store the plant on the flat and stable surface, conical part upwards.


Before starting the construction works, you will need following material and tools / machinery:

  • good quality adhesive-sealant (eg., Wurth Bond+Seal),
  • joint-pins M4x30 A2 or stainless steel screws DIN7337 M4x35 A2,
  • backfill: light soil (3-20 mm size sand or gravel),
  • PVC sewer pipes of required diameter,
  • PVC pipes or hose for air supply of required diameter,
  • electrical cable 3 x 1,5 kW, 220 V (in case when the air blower is installed outside next to the plant),
  • cable protection hose (in case when the air blower is installed outside next to the plant),
  • shovels, mini excavator bucket,
  • personal protective equipment (vest, gloves, helmet etc.).

Also water supply is necessary in construction site.


The location where the plant will be installed should be selected as described in the technical project. If technical project is not available, the location shall be selected in accordance with Construction Technical Regulations. The plant shall be installed in location where the sludge suction vehicle can easily service it. Also, do not install the plant in a place where the rain- and snow water accumulates.


Groundworks should be carried out in accordance with Construction Technical Regulations and technical project.


The excavation shall be excavated in two stages:

  1. The excavation is dig up by the excavator up to 20–30 cm of the altitude of the bed of excavation.
  2. The last 20–30 cm of ground shall be excavated manually, in order the plant will rest on the untouched ground.


Model People equivalent Hydraulic daily flow  Cover, D (mm)  Diameter (mm) Height (mm)  Inflow (mm) 
FEL-4 4 0,8 1000  1540 2150/2360/2530  500/710/880 
FEL-6 5 0,9  1000  1640  2200 710 
FEL-10 10 1,5  1000  1780  2690 900 
FEL-15 15 2,25  1000  2180 2800  900 
FEL-25 25 3,75  1000  2380 3700   1130


1. Low the plant into the excavation. Use lifting belts and an excavator bucket. Fasten the belts to the anchors (handles).

NOTE: The excavation must remain dry. When groundwater level is high, use a pump to remove the water from the excavation.

2. Backfill, i. e. light soil shall be compacted with water and / or using the soil compactor. It is prohibited to use different ground for backfill (eg., clay, construction waste, etc.)!!! At this stage it is necessary to ensure the level of the plant. Use a spirit level.

3. Join the plant with manhole. It is necessary to ensure watertightness. Use good quality adhesive-sealant and joint-pins or stainless steel screws. Distance between screws is about 15 cm. Join the manhole with the inspection lid too.

4. Fill the tank with water, and the excavation with the backfill by equal 20–30 cm levels.                                                                                                                       

5. Connect PVC sewer pipes to the inlet and outlet sleeves. The slope of the pipe should be 1-2 cm/m. Pipeline must be laid on the natural soil. Materials such as gravel, sand can be used for the base of pipeline.

6. Finish the backfilling up to designed altitude. For the upper layer of soil (20–30 cm) use compost. The upper layer of ground around the plant should be 5–10 cm higher than local area surface. Seed with the grass.


The maximum allowable distance between the air blower and plant can be 8–10 meters. So, there are two possibilities to install the air blower:

1. Indoor in non-living premises, where the distance between the house and the plant is up to 10 meters. Do not install the air blower in the living rooms, because the noise at the night time can be frustrating, also in dusty and humid places, because the lives of the diaphragms and valves may be shortened due to less suction air through easily blocked filter. When installing the air blower indoor, take care that it is well ventilated.

The ambient temperature cannot exceed + 40 ºC. The air blower installed indoor should not touch the walls, because it can lead to increased noise and vibration. The air supply pipeline must as short and straighter as possible. Underground air supply pipe should be installed with a slight slope toward the plant in order to prevent the condensation in the system. Avoid steep angles and bends in order to reduce the losses of air pressure.

2. Air blower is outside next to the plant, where distance between the house and plant is more than 10 meters. In this case the air blower must be protected from rain or snow waters, so:

use special box produced by Feliksnavis, UAB, do not install the box in a place where the rain- and snow water accumulates. Connect PE fitting which is located on the outer frame of plant with PE fitting which is located on the outer frame of box. Air supply pipe should be installed with a slight slope toward the plant in order to prevent the condensation in the system. Use underground pipe or hose made of PVC or other plastics. Also lay the underground electrical conduit cable to the electrical connection of the box.


When the plant has to be installed under low intensity vehicular traffic (vehicular loading under 1.5 tones), it is necessary to protect it from vehicle load. In this case it is necessary to install reinforced concrete plates. The plate should be mounted on the poles. The concrete plate cannot be bedded down on the plant; at least 5–10 cm of backfill shall be between the plate and plant. The size of plate is calculated by a designer (constructor) individually in each case, because it depends on vehicle weight and intensity.

The plant to be installed under vehicular traffic is equipped with extension shaft. 1 meter standard diameter is reduced to 0.6 meters. You can install plastic or cast iron sewer cover on top of it.


When the plant has to be installed in wet ground, where groundwater level is less than 2.5 meters from the ground surface, it should be anchored to reinforced concrete base or placed into well-sealed reinforced concrete rings. The force affecting the plant is specified in Table # 03 (page 14).

The size of reinforced concrete base shall be not less than the diameter of the plant at its widest point, thickness – not less than 20 cm. The plant cannot be supported directly to the concrete base. Make a 100-200 mm layer of backfill. In case if customer ignores this note, the manufacturer is not responsible for long-term physical or mechanical forces which can deform the plant.

Model Number of anchors (handles) Total lifting force (kg) Force acting on one anchor (kg) 
FEL- 4 2 2800 1400
FEL-6 2 3580 1790
FEL-10 2 3940 1970
FEL-15 2 6100 3050
FEL-25 2/4 13060 6530/3265

Look instalation video : 


Wastewater treatment plant can be started-up by the owner. There are two options to start-up it:

  • Natural grow up :

With this option microorganisms (i.e. activated sludge) have to grow up in wastewater treatment plant naturally. During installation works the plant had to be filled with water, the air blower connected to the plant, and electric power connected to the air blower. Switch on the air blower. You should observe turbulence (bubbles) in central chamber. The aeration process is starts. Within first week of operation supply wastewater into plant only in urgent cases (i.e. WC, shower). Within first month of the operation limit the quantity of household chemicals; use washing machine and/or dishwasher only if it necessary. I.e. during first month of operation the wastewater treatment plant shall receive more organic matter (WC, shower, sink) and less chemicals and large amount of water. From second month of operation the plant can be fully loaded without limiting ourselves in household. This option is the best when the average day temperature is no less than +10 ºC and at night time no less than 0 ºC, because cold season is not a good time for reproduction and decompose of microorganisms.

  • Bringing activated sludge

Switch the air blower on and aerate the plant for one day. Next day pour the activated sludge from a properly operating biological wastewater treatment plant: the amount of sludge – 5–10 % of the volume of the plant (see Technical parameters), concentration 4–6 g/l. Aerate the plant all next day. After this procedure you can supply wastewater into plant without limiting ourselves in household.

This option can be performed at any time of the year.

Watch the video how quickly and easily start the cleaning equipment of UAB Feliksnavis:

Daily operation

To maintain efficient daily operation of the plant, the following should be noted. Operating the plant is FORBIDDET TO :

  • Pour bacteria and other bio preparations (eg.: Septic Gobbler, Toilet Gobbler, BioActive, Bio 7 Fosses ). Such bacteria are intended for sewage or septic tanks, other anaerobic systems to eliminate the odour and decompose organic matter.
  • Discharge from water softeners (i.e. regeneration process wastewater) into the plant as the salts will significantly reduce the efficiency of wastewater treatment. In this case it is necessary to provide a separate drainage pipeline for process wastewater, bypassing the plant.
  • Discharge from condensing boilers.
  • Connect swimming pools, hot tubs etc. A large amount of water can wash the activated sludge from the system, and chlorine may affect the quality of the activated sludge.
  • Connect surface run-off, rainwater. Surface/rainwater should always be discharged to a rain drainage.
  • Put down to the toilet hygienic products, such as paper towels, sanitary pads, tampons, cotton pads, cotton swabs, diapers, wet wipes, rubber products (gloves, condoms) and other non-biodegradable waste.
  • Put down or pour to the toilet, sink or the plant itself the waste, which can block the pipes and/or all system, eg., cat litter, cigarette butts, garden chemicals and paints, automotive oil filters and engine oil, medical drugs, construction waste, sand, gravel etc.
  • Pour large quantity of oil and fat from fryers or pans. The allowable quantity of grease is the quantity remaining on the dishes and during the dish washing.
  • Even when the plant is not in use, it cannot be left empty! Ground pressure may affect the empty plant from outside, so it always must be filled with water.

Operating the plant is RECOMMENDED TO :

  • Limit chemical disinfectants which contain chlorine or active oxygen. Such disinfectants are marked with symbols Health hazard and Toxic.

  • Avoid a laundry day, and spread laundry through the week.
  • Do not exceed the doses of household chemicals and use it as subscribed on the package.
  • Use the same household cleaning chemicals, as the activated sludge will work more efficiently with familiar products.


How does it work?

Wastewater treatment plants treat biologically

The FEL plant consists of aeration chamber, secondary settler and air supply system (air blower and diffuser). The influent from the house first enters the aeration chamber. Its centre is equipped with small bubble diffuser. The supplied compressed air oxidizes organic pollutants. The circulation of activated sludge and pollutants takes place in aeration chamber. After start-up and a certain period of time bio-skin formed on the bio-load. When the circulating flow goes down, the concentration of the dissolved oxygen decreases, the bio-skin develop not only the processes of decomposing of organic pollutions, but also the processes of nitrification (an oxidation process of ammonia to nitrate) and denitrification (oxygen consumption from nitrates and nitrites). As a result, nitrogen concentration in the treated waste water is reduced. So, due to all these processes the treatment of wastewater takes place. Then treated wastewater and sludge fall into secondary settler. The separation process takes place: the sludge separated (precipitated) from wastewater. Then treated wastewater is carried over the scum baffle to the duct and goes out from the plant.

Biologically treated domestic wastewater is not harmful to the environment, so it can be discharged into flowing water bodies (rivers, canals, ditches). In absence of such possibility the treated wastewater can be discharged into the ground. Also it may be collected and used as technical water (lawn watering, car washing etc.).


Technical parameters

Model People equivalent Hydraulic daily flow  Standard dimensions Power consumption 
Pieces of air-blower V, m³

Control box d/h

H, mm D, mm D1, mm H1, mm Pipe, mm
FEL-4 4 0,8 2360* 1540 1000 710* 110 1,2 1 1,79 400/400
FEL-6 6 0,9 2340 1640 1000 850 1,73 1 1,95 400/400
FEL-10 10 1,5 2690 1780 1000 900 2,76 1 2,44 400/400
FEL-15 15 2,25 2800 2180 1000 900 5,04 1 3,67 500/400
FEL-25 25 3,75 3700 2380 1000 1130 160 7,32 2 6,73 800/400

* Other inflow depths are available for this model. Ask the manufacturer.

Troubleshooting and problem solving
Symptom/Problem Possible reason Remedial action
1. Air blower is running but little or no turbulence is observed in aeration chamber.
This could be due to few reasons:
a) blocked rubber diffuser a) disconnect air supply pipe, wash the rubber diffuser
b) leakage or air in air supply
pipe, hose or fittings
b) inspect pipes, hoses, especially at joints. Unblock as necessary
c) the condensate accumulates inside of air supply pipe or hose c) air supply pipe is laid without the slope. Relay as necessary
d) worn air blower parts d) inspect air blower membranes, valves and
diaphragm. Replace the parts or change into new one
2. A lot of white foam observed in aeration chamber. Foam climbs from the plant outside. NOTE: fresh white foam can be observed 1 –3 days from the startup of the plant. It is natural  phenomenon, meaning the growth of microorganisms a) excessive use of laundry
a) don’t use a household chemicals for some time. The situation will return to normal after a day or few
3. Heavy dark-brown foam on thesurface of aeration chamber a) activated sludge is very old a) make a jar test and de-sludge the plant, if  necessary 
b) the plant is full of excess 
4. Aeration chamber contents is greyish/blackish, may be felt slight septic odour a) the anaerobic processes take place in the plant, insufficient dissolved oxygen. See B a) – c). See. C a) – c).
5. Effluent is not clear, lowquality, contains brown/blacksludge particles, especially atpeak-flow a) excessive hydraulic load from laundry and/or other activities a) reduce the frequency of laundry to 1 load per day
b) the plant is full of excess
b) make a jar test and de-sludge the plant, if necessary
c) excessive layer of scum has
accumulated in the secondary
c) make a jar test and de-sludge the plant, if necessary
6. Grease balls are observed in the aeration chamber a) excessive pouring of fat into the drain from the kitchen a) do not use household chemicals for some time. The situation will return to normal after a day or few. If the fat balls remain, remove them
manually with the scoop
7. Thick dark sludge in secondary settler is being carried over the scum baffle a) low load of organic pollutions a) supply more organic matter (WC, shower, sink) and less chemicals and large amount of water into the plant for some time. The situation will return to normal after a few days
b) excessive load of organic
situation will return to normal after a few days b) excessive load of organic pollutions
b) Too many people use the plant. You should appeal to designers for selection of correct size or additional devices
c) influent before entering the
plant overstays on its way, it
being to fermented
c) inspect pipeline and control shafts for blockages. Unblock if necessary
d) excessive use of acid
chemicals, low pH of the
d) don’t use an acid chemicals for some time. The situation will return to normal after a day or few
e) the plant receives nondomestic wastewater e) for commercial application you should appeal to designers for or additional devices
f) the air blower is running not constantly f) inspect air blower membranes, valves and
diaphragm. Replace the parts or change into new one. Don’t use a timer, if you do so
Technological parameters (certificates)

Biological wastewater treatment plants FEL meet requirements of standard EN 12566-3+A2:2013 Small wastewater treatment systems for up to 50 PT - Part 3: Packaged and/or site assembled domestic wastewater treatment plants and Construction Product Regulation (CPR) No. 305/2011.

Biological wastewater treatment plants were tested in SZU Engineering Test Institute (Brno, Czech Republic).


Certificates and declarations of products see here >>>

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