Nutrient Analysis

NITROGEN



Nitrogen Species in Wastewater



In wastewaters, nitrogen may be found in four forms: organic nitrogen, ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen.



The nitrogen present is primarily combined in proteinaceous matter and urea as organic nitrogen. Decomposition by heterotrophic bacteria, known as ammonification, readily converts organic nitrogen to ammonia nitrogen. Ammonia nitrogen may exist in aqueous solution as either ammonium ion or unionized ammonia. The relationship between the two forms is pH dependent.



Ammonium is predominant at any pH less than 7.0. Unionized or free ammonia in concentrations above 0.2 mg/l, has been shown to be fatal to several species of fish.



Nitrate nitrogen is the most highly oxidized form of nitrogen and is an important nutrient for algae growth and, when present in excessive quantities, may be responsible for promoting eutrophication in streams and lakes. Most of the nitrogen in municipal and many industrial wastewaters occur in the form of ammonium (NH4+), which is in pH dependent equilibrium with ammonia (NH3+) or is bound in urea or other organic molecules, while nitrates are formed through the nitrification reaction which is ubiquitous in nature in the presence of oxygen.



Under certain conditions, up to 8 percent of nitrate is converted to N2O; favorable conditions for its production are low COD/NO3, short solid retention time, and low pH.



ORTHOPHOSPHATES



PHOSPHORUS



Average total phosphorus concentrations in raw influent ranges from 5–20 mg P/L, of which 1–5 mg P/L is the organic fraction and the remainder being inorganic orthophosphates. After secondary treatment, approximately 80% of the total phosphorous is in orthophosphate form.



Phosphorous is essential in key phases of plant growth. It hastens ripening, encourages root growth and increases resistance to disease. Thus it is a key promoter of algal blooms in the event of eutrophication of a water body.



Biological Phosphorous Removal



Biological phosphorous removal involves sequestering phosphates from within the mixed liquor to within the cells of specific bacterial populations, and subsequently removing these cells and their stored polyphosphates from the system through sludge wasting. This is known as Enriched Biological Phosphate removal.



Chemical Phosphorous Removal



Traditionally phosphorus is removed by the addition of coagulants to wastewater, although the development of EBPR has led to the development of biological removal processes. Chemical precipitation is used to remove the inorganic forms of phosphate by the addition of a coagulant (lime, aluminium salts or iron salts).



The selection of a coagulant depends on a number of factors:



(i)         influent P concentration;



(ii)        influent suspended solids concentration and alkalinity;



(iii)       cost of the chemical used at treatment plant;



(iv)       availability and reliability of chemical supply;



(v)        sludge handling facilities at plant or cost of new system;



(vi)       disposal method of sludge and cost (if sludge is disposed to agricultural land then iron or aluminium sludges are less favoured than those treated with lime)



(vii)      compatibility with other unit processes used at the plant; and



(viii)     the potential environmental impact of using coagulants.



SULPHUR COMPOUNDS



Sulphur is a relatively abundant compound in the natural environment, with seawater being the largest reservoir of sulphur in the form of sulphate. Other sources include sulphur-containing minerals (such as pyrite, metal-sulphide salts, and chalcopyrite CuFeS2), fossil fuels, and organic matter.



It is an essential element for microorganisms as it is an important component of amino acids (cystine, cysteine, and methionine), cofactors (thiamine, biotin, and coenzyme A), ferredoxins and enzymes (-SH groups).



Problems associated with Sulphur reducing bacteria



Biocorrosion or microbially influenced corrosion (MIC) is the deterioration of metals by microorganisms, which affects the infrastructure of commercial industry as well as the water and wastewater treatment industries. Pitting corrosion develops under strictly anaerobic conditions and involves sulphate-reducing bacteria such as Desulfovibrio desulfuricans. The pits are filled with iron sulphide. Corrosion may be inherently linked to the metabolism of sulphate reducers or may be due to hydrogen sulphide or iron sulphides. The development of biofilms on pipe surfaces leads to anaerobic conditions that are ideal for the growth of obligate anaerobic sulphate-reducing bacteria. These conditions are achieved when the biofilm reaches a certain thickness, varying from 10 to 100 mm.



Hydrogen sulfide is the most commonly known and prevalent odorous gas associated with domestic wastewater collection and treatment systems. It has a characteristic rotten egg odour, is extremely toxic, and is corrosive to metals such as iron, zinc, copper, lead and cadmium. Hydrogen sulfide is also a precursor to sulfuric acid formation, which corrodes lead-based paint, concrete, metals and other materials.