Nitrification Article - Water & Sewerage Journal
Successful Nitrification By Bioaugmentation.
Within the UK, municipal waste water treatment has historically concentrated on preliminary, primary and secondary treatment procedures only. Preliminary treatment removes constituents that may cause maintenance or operational problems within the treatment process and frequently include screening, grit removal and flotation processes. Primary treatment removes a portion of the suspended solids and organic matter in the wastewater and mainly serves as a precursor to the secondary treatment. The main aim of secondary treatment is the removal of biodegradable organics.
Advanced or tertiary treatment involves processes to remove nutrients, toxic compounds and increased amounts of organic material and suspended solids. The main nutrients of concern are nitrogen and phosphorous which cause eutrophication in confined bodies of water. In addition, the presence of nitrogen can deplete oxygen levels in the receiving water. Nutrient removal can be effected through chemical or biological processes and are often coupled with secondary treatment. The most widely used process to remove nitrogen is biological nitrification/denitrification. Biological nitrification is the process of choice to treat ammonia as the process has a high removal efficiency potential, is relatively easy to control, requires only a small footprint and is financially economical. The increasingly stringent discharge levels being set for ammonia both here in the UK and elsewhere in the world will require effluent treatment operators to either optimise or increase the capacity of their existing nitrification processes or to install such a process when one does not already exist, as part of the treatment regime.
Optimisation of the nitrification process requires an understanding of the biological processes and organisms involved. The conversion of ammonia to nitrate is a two-step process carried out by two groups of bacteria belonging to the genus Nitrosomonas and Nitrobacter. Neither of these bacteria can metabolise organic carbon to meet their energy requirements. Instead they derive their energy for growth from the oxidation of inorganic nitrogen compounds such as ammonia and nitrite. As a result of their unique metabolic pathways their growth rates are extremely slow and typical doubling times are in days compared to less than an hour for organisms that metabolise organic carbon. The slow growth rates of the nitrifying bacteria make them very sensitive to a wide range of physical and chemical parameters. Consequently a thorough understanding of the operational parameters and regular monitoring of the process is vital to ensure maximum nitrification is achieved. Optimal nitrification occurs when the temperature is about 300C, the levels of dissolved oxygen and available alkalinity are high, the pH is about 8.0, salinity is low, BOD levels are low, heavy metals are absent and biomass is attached to solid media, there is sufficient retention time and adequate mixing. The key parameters to monitor are temperature, pH ammonia and nitrate levels. If any problems are noticed with the nitrification process then additional monitoring needs to be done on the dissolved oxygen level, available alkalinity level and BOD levels. If serious problems with nitrification are still encountered more involved monitoring is needed to find out the chemical composition of the effluent.
Nitrifying bacteria are present in almost all aerobic biological treatment plants. However their numbers are usually very small. In a typical activated sludge process, which is a combined carbon oxidation and nitrification process, the nitrifying population is low and usually less than 3% of the total biomass. The quickest way to boost cell numbers is by Bioaugmentation. Bioaugmentation with nitrifying bacteria is a simple process requiring the addition of a highly active preparation of nitrifying biomass to the inlet of the aeration tank. The nitrifying biomass will then rapidly colonise the nitrification zone of the aeration tank.
Bioaugmentation with nitrifying bacteria can also help in shock recovery situations. The time required to regain full nitrification following a physical or chemical shock can often run into weeks and months. Bioaugmentation has the potential to restart activity within days.
Bioaugmentation also helps when additional nitrifying activity is required. Introducing additional capacity by installing extra aeration tanks is both costly and subject to space constraints. Bioaugmentation has zero capital costs and no space issues.
The main requirement for a successful bioaugmentation programme is the availability of active nitrifying biomass. Cleveland Biotech Ltd can supply nitrifying bacteria with an activity of 5000 mg N/oxidised/h/Kg product. Over the years Cleveland Biotech Ltd has supplied nitrifying biomass to both municipal waste water treatment plants and industrial treatment plants both in the UK and around the world including Europe, North and South America and Australia. Nitrifying products from Cleveland Biotech Ltd have also been successfully used in odour control of emissions containing ammonia and treatment of both domestic and industrial fish ponds.
An example of where this has been put into practice is a large effluent treatment plant which was experiencing recurring problems achieving and maintaining adequate rates of nitrification. As part of their assessment of a bio-augmentation approach to solve the problem, the plant operators ran a detailed trial of Cleveland Biotech's AMNITE NS5000 - concentrated, pre-grown nitrifying paste. The results of the trial can be seen below. In excess of 70% of the ammonia was removed within 3 hours of the addition of the nitrifying bacteria. On the strength of this result, Cleveland Biotech was asked to calculate what quantity of this product would be required to re-establish nitrification in the plant. Such an assessment was performed based upon hydraulic throughput, influent ammonia levels, total plant volume and required ammonia discharge levels. A one-off dose of bacteria to re-establish the total nitrifying capacity in a single step, was found to be cost prohibitive. However assuming a conservative mean generation time for the bacteria and from this allowing for a calculated period of time for full nitrification to become re-established by growth of the new bacteria within the plant, a cost effective solution to recover ammonia removal was arrived at.
For further information about nitrification products from Cleveland Biotech Ltd please visit www.clevebio.com or email sales@clevebio.com or call Dr Sundaram Manian on 01642-606606.
Dr Sundaram S Manian, B.Sc. (Chemistry), M.Sc. (Biochemistry), Ph. D. (Microbiology)
