Biofilters (Biorotors) and secondary settling tanks

Small WWTP reconstruction with rotating disk biofilters, Melnikovo settlement

The practice introduces an approach to the reconstruction of existing treatment facilities, constructed in past century as per typical project. The reconstruction involves application of drum-type biological oxidizers with attached and free-floating microflora of oxidizing microorganisms at the biological treatment stage.

Solution to which problem

Melnikovo settlement is located in Priozersk district of Leningrad region. As of 2013, Melnikovo settlement population is 1746 people, of which 1250 people live in apartment buildings equipped with water supply and sewage systems, 496 people live in separate houses and use stadpipe and waste pits. The development plan for the settlement includes constructing living spaces with amenities for 160 people. In addition to the population, water in Melnikovo settlement is used by community facilities and enterprises (outpatient clinic, kindergarten, school, groceries, pharmacy, boiler house).

The settlement has central water supply systems, domestic sewage systems, heat supply systems, power supply systems. Domestic wastewater of the settlement is collected by a local sewerage pump station (SPS) through sewerage systems and supplied to the wastewater treatment plant (WWTP). Design capacity of treatment facilities built in 1970s is 1400 m3/day. The actual consumption of treated wastewater is around 340–450 m3/day. Discharge of treated wastewater from treatment facilities is performed into the Vuoksa river.

Technical conditions

The wastewater treatment plant in Melnikovo settlement, Leningrad region, was designed in the early 1970s based on tank units for biological treatment of wastewater as per typical project TP 902-2-293. Wastewater treatment is performed in aerotanks with pneumatic aeration without primary settling with separate aerobic fermentation (mineralization) of surplus activated sludge, advanced wastewater treatment is performed in filters.

Treatment technology implemented based on WWTP was outdated, particularly the removal of nitrogen and phosphorus compounds wasn't implemented in accordance with the current requirements. There were no mechanical screens in bar screen building, wastewater treatment for large floating wastes removal was not performed. Tangential sand traps, operating in non-uniform load mode, were missing the particles of mineral origins with a size of more than 0.25 mm. Sand, removed from sand traps, contained significant organic constituents. Sand site, to which sediments settled in sand traps are regularly discharged, was overgrown with reed, drain water removal system was out of operation, dried sand was not hauled away. Of all three-section tank units structures, only one aerotank, one secondary settling tank and sludge densifier were under operation. The remaining section was half-filled with wastewater, the aeration system was operated in critical condition and required replacement. Due to the imperfections in the return and excess sludge collection and removal system, the secondary settling tank did not provide reduction of suspended matters concentration according to the design values. Partially dead sludge was returned to aerotank.

Aerobic stabilizers, additional treatment system and chlorinator house were removed from service due to critical condition. Sludge drying beds were outgrown with reed, sludge water removal system was out of operation, dried sediment was not hauled out.


The wear of communications and technical condition of tanks, as well as operational issues with WWTP in Melnikovo settlement and necessity of expanding the station, resulted in a need for reconstruction. The following requirements to the treatment system were listed:

  • possibility of operating WWTP without the maintenance staff present at all times;
  • limiting the usage of electric power for heating the industrial building during winter time;
  • reliable operation, resistance to short-time hydraulic overloads and single loads by means of averaging for 12–24 hours and simultaneous dilution of wastes with circulation water;
  • low energy consumption due to usage of in-situ aeration conditions (lack of air blowers), simple design;
  • self-regulation of biofilm buildup and breakaway, opportunity for sustained interruption in the supply of wastes and power.

The design for the reconstruction was done in 2014–2015. Construction and commissioning – 2016.

The treatment facilities have been operated since 2016 and all the installed equipment have been working the whole operating time of the complex.


For wastewater treatment in Melnikovo settlement, the construction involved the wastewater biological treatment complex “AllianceBio-500” with capacity of 500 m3/day and immersion rotating biofilters (biorotors) technology application.

The advantages of biological treatment with rotating disk biofilters (biorotors) technology application under conditions of distant underpopulated areas:

  • resistance of biological treatment process using biorotors to inhibitory external influences, characteristic for small settlements - high load imbalance, low temperatures of wastewater.
  • high treatment efficiency for organic contamination and ammonia nitrogen oxidizing;
  • low power consumption;
  • easy servicing - high-skilled professionals are not required;
  • to provide the advanced treatment for nitrogen the reverse recycling into denitrifier is arranged;
  • increasing biocoenosis (excess sludge) is continuously washing out during rotation and removing at secondary setting stage.
  • industrial building heating during winter time is performed using heat pump. Treated wastewater acts as a heat source.

Wastewater treatment plant complex “AllianceBio-500” based on rotating biological rotors is designed as a single process complex of process units, systems and equipment, providing a single process of multi-stage treatment and disinfection of wastewater.

The main treatment stage of rotating biorotors technology is performed in multi-sectional biozone, in which denitrifier, settling tank, biozone with half-filled biorotors and secondary settling tank are structurally combined. Wastewater biological treatment process is performed in biofilm at a surface of slowly rotating disks - without air blowers use. Excess biofilm is settled in the secondary settling tank. Reactants treatment is used to remove phosphorus. Treated water is disinfected at UV-disinfection unit.

Excess sludge that is formed during the treatment process is removed into the sludge tank-acidifier and supplied further by the pumps for dewatering.

The wastewater treatment plant complex “AllianceBio-500” operates reliably, providing the level of treatment according to the standard characteristics for discharge into the 1st category fishery basin.

Efficiency of wastewater treatment using immersion rotating biorotors is 95% for COD/BOD, 98% for suspended matters, 90% for total nitrogen and 95% for total phosphorus. Efficiency of wastewater treatment using immersion rotating biorotors is confirmed by laboratory analyses of treatment facilities in operation.

The automated process control system allows to optimize maintenance expenses. Regular maintenance is performed by 1 person.

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