Treatment process - VFL® Wastewater treatment plants

The wastewater treatment technology uses a continuous-flow activated sludge process with biological nitrogen and phosphorus removal, which combines the following processes in a single tank: mechanical pre-treatment, excess sludge collection, biological treatment using a low-loaded activated sludge process, separation of the treated water from activated sludge in the final clarification chamber, flow balancing of fluctuating inflow of wastewater in the retention chamber. The treatment process consists of several technological processes. Raw wastewater flows into the non-aerated activated sludge chamber with anaerobic and anoxic zones creating and admixture with the recirculated activated sludge, the mechanical pre-treatment of inflowing raw wastewater and the decomposition of coarse impurities, denitrification and accumulation of readily degradable organic contamination is taking place in the non-aerated activated sludge chamber, which is divided by inner partition walls to create a vertical flow labyrinth, where internal circulation is established.

VFL - Wastewater treatment plantsFurthermore, allowing the admixture flow gravitationally into the aerated activated sludge chamber with includes fine-bubble diffusors. In oxic conditions the biological degradation of organic contamination, nitrification and phosphorus uptake is taking place. The activated sludge flows into the final clarification chamber, where the activated sludge is separated from the treated wastewater. The treated wastewater is discharged into a water flow, infiltrated or recycled and the separated activated sludge is recirculated by air-lifts.

A flow regulator is installed at the water level in the final clarification chamber which controls the outflow in order to maintain the water level between the normal and maximum level in the tank (integrated retention chamber).

The pressurized air is supplied by blowers for aeration of the activated sludge chamber and for recirculation via air-lift pumps. The recirculation and aeration is controlled by a microprocessor control unit which also enables the wastewater treatment plant to work in various modes depending on the loading.

Main features and advantages

High effluent quality

The treatment efficiency of the biological treatment step was tested in Testing Institute for Wastewater Technology GmbH (PIA GmbH) Aachen, Germany with the following results:

Parameter Testing Institute for Wastewater Technology GmbH (PIA GmbH) Aachen, Germany Treatment efficiency Concentration in the effluent
Chemical Oxygen Demand (COD) 94.4% 45 mg/l
Biochemical Oxygen Demand (BOD5) 98.2% 7 mg/l
Total Suspended Solids (TSS) 97.2% 12 mg/l
Ammonium Nitrogen (N-NH4) 99.5% 0.2 mg/l
Total Nitrogen (Ntot) 93.2% 5.6 mg/l
Total Phosphorus (Ptot) 93.3% 0.6 mg/l

Handling of highly fluctuating flows In the final clarification chamber, a flow control device is incorporated, which enables the usage of the volume of the integrated retention chamber in all the compartments and tanks of the wastewater treatment plant during peak flows and thus protect against the overloading of the plant. This makes possible a more effective tertiary treatment and water recycling, because the surface load and the dosing amount of chemicals can be maintained constant even at peak flows. The discharged treated wastewater does not clog the filtration layer in the in the subsurface dispersal system or the treated water has a stable quality which enables the using of reuse schemes. Low power consumption The energy efficiency is very high in comparison with the large wastewater treatment plants (data from 369 WWTPs*) and with other small WWTPs**:

Wastewater treatment plant size Specific energy consumption
[kWh/PE.year] [kWh/kgCODrem]
WWTPs up to 2000PE* 105 - 472 1.35 – 3.39
WWTPs in the range of 2000PE – 100 000PE* 27.4 -47.9 0.55 – 1.10
VFL systems in the range of 75PE – 900PE 35 - 47 0.88 – 1.18

*Source: ENERWATER - Standard method and online tool for assessing and improving the energy efficiency of waste water treatment plants, Deliverable 2.1 Study of published energy data– Final version – 2015-09-30

Small wastewater treatment plants Specific energy consumption
[kWh/PE.year]
SBR systems for 4PE-8PE ** 93
MBR system for 4PE-8PE** 90 - 140
Fixed bed systems for 4PE-8PE ** 157
VFL systems for 4PE-8PE 43.8

** Source: Daten des BDZ e.V. und PIA, Aachen – Durchschnittlicher Stromverbrauch nach Anlagentyp, Dorgeloh & Defrain (2008).

Low excess sludge production
The excess sludge production is very low due to the

  • Vertical stratification of sludge blanket in the vertical flow labyrinth.
  • Decomposition of excess sludge under anaerobic-anoxic conditions.
  • Mineralization of excess sludge in oxic conditions.
  • Alternating of anaerobic-anoxic-oxic conditions during the aeration, recirculation and mixing phase of operation.
  • High sludge age 60-120 days.
Small wastewater treatment plants Specific production of excess sludge
[kg SS/kg BOD5rem] [g SS/PE.day]
Conventional systems* 0,69 35 – 85
VFL wastewater treatment plants 0,21 12

*Sludge Reduction technologies in Wastewater Treatment, IWA Publishing (2010)

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