Biological Process

The biological engineering of Envirocycle® products is based on well-known principles in the wastewater field, applied in an innovative new way. In a sentence, the system can be described as a hybrid ?xed ?lm-suspended growth extended aeration wastewater treatment system with a two stage biological process to optimize denitrification.

The Envirocycle® G7 unit removes nitrogen using biological processes, such as ammonification followed by nitrification and denitrification. In ammonification, organic nitrogen (proteins and peptides) is decomposed to ammonia or ammonium ions. About 80% of the ammoni?cation take place in the sewer lines before the wastewater enters the Envirocycle® G7 unit and the balance is ammonified in the first compartment. The ammonification is followed by nitrification. In nitrification, ammonia is removed biologically by a two-step process in which the ammonia is oxidized to nitrite, and the nitrite is oxidized to nitrate according to the following formulas 1, 6, 10:

NH3 + O2 + CO2 + HCO3- + Microbes ==>
New Microbes + NO2- + H+ +H2O
NO2- + O2 + CO2 + HCO3- + Microbes ==>
New Microbes + NO3-

The nitrification is affected by temperature, pH, dissolved oxygen (DO), alkalinity, contact time, and mean cell residence time 2, 4, 10. The temperature and pH are not specifically controlled in the Envirocycle® G7 unit. The temperature is normally kept between 70 to 90°F by the microbial activity and some added heat from the air blower. The pH is typically between 7 and 8.5 in the Envirocycle® G7 unit, since no chemicals are added to any of the compartments. Therefore, both the temperature and the pH fall well within the optimum range for nitrification.

An energy efficient air blower supplies air to the two aerobic compartments in the tank to keep the dissolved oxygen above 3 mg/l. The conversion of ammonia to nitrates requires 4.57 kg of oxygen per kg of ammonia converted 9, 11, 12. Furthermore, it requires about 7 mg of carbonate alkalinity per mg of ammonia nitrogen6. Typically, the alkalinity concentration in the tap water is enough to convert all the ammonia to nitrates, but in some cases an alkalinity source has to be added.

Nitrate formed during nitrification is removed by heterotrophic organisms under anaerobic conditions through conversion to gaseous nitrogen species through denitrification 10, 11, 12. In this process, nitrate first is reduced to nitrite and then to nitric oxide (NO), followed by nitrous oxide (N2O) and nitrogen gas (N2). This process requires a carbon source 4. In the Envirocycle® G7 unit, the wastewater exiting the two stage aerobic section, which is high in nitrates and low in car bon, is recirculated back to the first suboxic chamber where it mixes with the raw wastewater, which is high in carbon. Denitrification requires 5-6 mg of BOD per mg of

Nitrate-Nitrogen removed, and it produces about 3 mg of carbonate alkalinity per mg of Nitrate-Nitrogen removed.

Nitrate-Nitrogen removed, and it produces about 3 mg of carbonate alkalinity per mg of Nitrate-Nitrogen removed.

The biodegradable organic carbon, that causes BOD5, is converted to carbon dioxide and settleable biomass by heterotrophic organisms10. These microorganisms require oxygen. The process is referred to as aerobic digestion and can be expressed by the following equation5, 9.

Microbes
Organic Matter + O2 + Nutrients ==>
New Microbes + CO2 + H2O

The aerobic digestion takes place in the second and third chambers of the Envirocycle® G7 unit. The Envirocycle® G7 unit utilizes a combination of an attached and suspended growth process. The attached ?lm is growing on a biomedia and the suspended growth is created by mixing and recirculation of sludge. This combination results in a treatment e?ciency that exceeds the individual performance of an attached or suspended growth process.

The aerobic digestion of organic matter is mainly a?ected by dissolved oxygen, pH, temperature, mixing, and solids retention time. The design of the Envirocycle® G7 unit optimizes these parameters for maximum BOD5 and nitrogen removal3, 4, 5, 8.

The fourth chamber is the clari?er where ?nal setiling of suspended solids and clari?cation of the e?uent is taking place. The tank design is optimized with respect to the following parameters: waste water ?ow rate, sludge setiling rate, sludge removal, surface area, tank depth, over?ow rate, inlet device, and tank con?guration7. It is designed for optimum performance without any chemical addition.
The settled solids are recirculated back to the ?rst chamber.

The ?fth chamber is an e?uent storage area which can be equipped with an optional gravity-?ow UV disinfection unit. This ?nal chamber also serves as a reservoir for water reclamation in uses such as irrigation.

Reference

  • “Design of Municipal Wastewater Treatment Plants Volume I”, WEF Manual of Practice No. 8/ASCE Manual and Report on
    Engineering Practice No. 76 (1992).
  • “Design of Municipal Wastewater Treatment Plants Volume II”, WEF Manual of Practice No. 8/ASCE Manual and Report on
    Engineering Practice No. 76 (1992).
  • “Operation of Municipal Wastewater Treatment Plants Volume I”, Manual of Practice No. 11 Fifth Ed., WEF (1996).
  • “Operation of Municipal Wastewater Treatment Plants Volume II”, Manual of Practice No. 11 Fifth Ed., WEF (1996).
  • “Operation of Municipal Wastewater Treatment Plants Volume III”, Manual of Practice No. 11 Fifth Ed., WEF (1996).
  • “Nutrient Control”, Manual of Practice No. FD-7, Water Pollution Control Federation, Washington, D.C. (1983).
  • “Clari?er Design”, Manual of Practice FD-8, Water Pollution Control Federation, Washington, D.C. (1985).
  • “Wastewater Biology: The Microlife”, A Special Publication, WEF, Alexandria, Virginia (1990).
  • “Aeration”, WEF Manual of Practice FD-13/ASCE Manuals and Reports on Engineering Practice No. 63 (1996).
  • “Wastewater Biology: The Life Process”, A Special Publication, WEF, Alexandria, Virginia (1994).
  • “Wastewater Disinfection”, Manual of Practice FD-10, WEF, Alexandria, Virginia (1996).
  • “Comparison of UV Irradiation to Chlorination: Guidance for Achieving Optimal UV Performance”, Project 91-WWD-1, Water Environment Research Foundation, Alexandria, Virginia (1995).