Question

1-Describe the Secondary Anaerobic Digestor process in wastewater treatment plant.

2-Describe the Nitrification Tower process in wastewater treatment plant.

Answer 1

1.

The AD process can be further divided into four stages: pre-treatment, digestion, gas upgrading and treatment for re-use of digestates and side flows.

The level of pre-treatment depends on the type of feedstock. For example, liquid wastewaters, manures are sludges need to be mixed, whereas municipal solid wastes (MSW), grasses and botanic wastes are pre- sorted and shredded.

The digestion stage takes place in the AD reactor(s). There are many types of digesters with different temperature, mixing devices, etc. The digestion can be either dry or wet depending on the solid content. Thus, the feedstock can be mixed with waters and other appropriate liquid wastes such as sewage sludge liquid organic wastewaters or re-circulated liquid from the digester effluent.

‘Mainstream’ digestion can perform ideally as a wastewater pre-treatment operation, removing up to 90% of COD with less than 20% of the equivalent excess sludge production.

Biogas produced during the digestion stage requires removal of hydrogen sulphide and water vapour. These can damage boilers or engines and should be removed. Removal of carbon dioxide will be required if the gas is to be used as natural gas or vehicle fuel. Choice of system is project based but efficiency of energy conversion should not be forgotten e.g. steam (95%), CHP (55%), engine (40%), fuel cell (50%+). Flaring is a safety inclusion, but otherwise does not provide reuse.

The AD process is a naturally occurring adiabatic biochemical process of decomposition and decay, by which organic matter in any bio treatable condition is broken down to its end products under anaerobic conditions.

Anaerobic microorganisms digest the organic materials, in the absence of oxygen, to produce methane and carbon dioxide and other end-products under ideal conditions. The biogas produced in AD plant usually contains hydrogen sulphide (H2S), ammonia (NH3) and carbon dioxide (CO2), as well as trace amounts of other gases. Concentrations vary with the feedstock constituents.

The science underlying AD can be complex, and the process is best understood if split into the three main stages: hydrolysis, acidogenesis and methanogenesis. The AD cultures are well established from microbiology studies over the years but ongoing development of new cultures is necessary (see cited references).

During the hydrolysis, the fermentative bacteria convert the insoluble complex organic matter, such as cellulose, into soluble molecules such as fatty acids, amino acids and sugars.

In the second stage, acetogenic bacteria, also known as acid formers, convert the products from the first stage into simple organic acids, carbon dioxide and hydrogen. The principal acids produced are acetic acid, butyric acid, propionic acid and ethanol.

Finally, methane is produced during methanogenesis by bacteria called methane formers. The acetate reaction is the primary producer of methane because of the limited amount of hydrogen available. It is important to note that some organic materials, such as lignin, remain effectively undigested, as of course do non-organic inclusions within the waste. This can bias feedstock choices if considering high bio-energy production performance.

2.

Bacteria remove ammonia nitrogen from wastewater by a two step biological processes: nitrification followed by denitrification to covert it finally to gaseous nitrogen. In this gaseous form N2 is inert and does not react with the wastewater itself or with other constituents present in wastewater. Since, treated wastewater is likely to be saturated with molecular nitrogen; the produced N2 is simply released to the atmosphere. These two steps involved require different environmental conditions and hence generally they are carried out in separate reactors. Nitrification It has important role in nitrogen removal from wastewater during treatment. The biological conversion of ammonium to nitrate nitrogen is called Nitrification. It is autotrophic process i.e. energy for bacterial growth is derived by oxidation of nitrogen compounds such as ammonia. In this process, the cell yield per unit substrate removal is smaller than heterotrophs. Nitrification is a two-step process. In first step, bacteria known as Nitrosomonas can convert ammonia and ammonium to nitrite. These bacteria known as nitrifiers are strictly aerobes. This process is limited by the relatively slow growth rate of Nitrosomonas. Next, bacteria called Nitrobacter finish the conversion of nitrite to nitrate.

Nitrosomonas and Nitrobacter use the energy derived from the reactions for cell growth and maintenance. Some of ammonium ions are assimilated into cell tissues. Neglecting this ammonium ion used in cell synthesis the O2 required to oxidize ammonia to nitrate is 4.57 mg O2/mg ammonium nitrogen. If the ammonium used in cell, O2 required is considered it is 4.3 mg O2/mg ammonium nitrogen and about 7.14 mg of alkalinity is needed to neutralize the H+ produced.

Nitrification may be used to prevent oxygen depletion from nitrogenous demand in the receiving water. Nitrification requires a long retention time, a low food to microorganism ratio (F/M), a high mean cell residence time (MCRT), and adequate alkalinity. Wastewater temperature and pH affects the rate of nitrification.

Nitrifying bacteria are sensitive organisms. A variety of organic and inorganic agents can inhibit the growth and action of these organisms. High concentration of ammonia and nitrous acid can be inhibitory. The effect of pH is also significant with optimal range of 7.5 to 8.6. The system acclimatize to lower pH can also work successfully. The temperature also has considerable impact on growth of the nitrifying bacteria. Dissolved oxygen concentration above 1 mg/L is essential for nitrification. Below this DO, oxygen becomes the limiting nutrients and nitrification slows down or ceases. Denitrification In some applications, such as discharge of effluent into enclosed water bodies or recycle to water supplies, nitrification may not be sufficient. When nitrogen removal is required, one of the available methods is to follow biological nitrification with denitrification. Denitrification is accomplished under anaerobic or near anaerobic conditions by facultative heterotrophic bacteria commonly found in wastewater. Nitrates are removed by two mechanisms: (1) conversion of NO3 to N2 gas by bacterial metabolism and (2) conversion of NO3 to nitrogen contained in cell mass which may be removed by settling. Denitrification occurs when oxygen levels are depleted and nitrate becomes the primary electron acceptor source for microorganisms. Nitrate, N03 - Nitrite, N02 - Nitric oxide, NO Nitrous oxide, N2O Nitrogen, N2 Denitrifying bacteria are facultative organisms, they can use either dissolved oxygen or nitrate as an oxygen source for metabolism and oxidation of organic matter. This is carried out by hetetrophic bacteria such as pseudomonas, spirillum, lactobacillus, bacillus, microaoccus, etc. For reduction to occur, the DO level must be near to zero, and carbon supply must be available to the bacteria. Because of low carbon content is required for the previous nitrification step, carbon must be added before denitrification can proceed. A small amount of primary effluent, bypassed around secondary and nitrification reactor can be used to supply the carbon. However, the unnitrified compounds in this water will be unaffected by the denitrification process and will appear in effluent. When complete nitrogen removal is required, an external source of carbon-containing no nitrogen will be required. The most commonly used external source of nitrogen is methanol. When methanol is added the reaction is N03 - + 5/6 CH3OH ½ N2 + 5/6 CO2 + 7/6 H20 + OH.

For treatment plant above 3 mg/L of methanol is required for each milligram per litre of nitrate, making this process an expensive. Alkalinity is generated in this process. Denitrification can be carried out as attached growth (anaerobic filter) and suspended growth process