Page 58 - March 2020 - WT Site

Optimal Nutrient Ratios for Wastewater Treatment

Continuously operating process measurement devices have demonstrated that they are indispensable aids to
achieving greater transparency and reliability in wastewater treatment. The article describes the causes and
effects of unfavourable nutrient ratios, & measures to be taken to deal with them.
By Michael Winkler

o be able to comply with the legal requirements on treated NH -N and the NH -N from the inflow are converted to nitrite,
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wastewater, plant operators must control the treatment which in turn is converted to nitrate (nitrification). The nitrogen
Tprocess carefully, so that they can intervene promptly to compounds that are not biodegraded in the activated sludge are
prevent ¾ limit values from being exceeded. Besides chemical converted under anoxic conditions (absence of dissolved O )
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and physical methods, wastewater treatment is essentially based to elementary nitrogen (denitrification). This escapes into the
on ¾ biological treatments by ¾ microorganisms in activated atmosphere as N .
sludge. Knowledge of the ¾ nutrient requirements and the 2
composition of the activated sludge are therefore needed if the ¾ Nitrogen compounds are determined as NH -N, NO -N,
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plant is to operate at maximum efficiency. NO -N and TN (total nitrogen, which is important for balancing
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and outflow checks).
Nutrients in Activated Sludge
Phosphorus
A balanced nutrient ratio is essential if the microorganisms are
to function at maximum efficiency. The most important of these The P load in the inflow of a wastewater treatment plant is made
nutrients are carbon, nitrogen and phosphorus. up of orthophosphate-phosphorus (PO -P), polyphosphates and
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organic phosphorus compounds. Together, they give the sum
Carbon parameter ‘total phosphorus’ (P ).
tot
Carbon is the principal component of the organic substances During biological wastewater treatment, polyphosphates
found in wastewater. It is biodegraded by the microorganisms in and organically bonded phosphorus are converted to
activated sludge under anaerobic conditions (bio-P), in an anoxic orthophosphate.
environment (denitrification zone) and in the aerated part of the
biological stage (nitrification zone). The microorganisms use the
car- bon compounds to build their cell structures and to generate Organic compounds + O + Nutrients
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energy. Microorganisms
New cell material + CO + H O
¾ Carbon compounds are determined as COD, BOD or TOC 2 2
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Nitrogen The P demand of the organisms is due to the special role of
phosphorus in their energy metabolism. P is needed to form
In the inflow of wastewater treatment plants, nitrogen is present in the cell membrane and DNA. Some of the phosphorus in
organically bonded form (organic N) and as ammonium nitrogen wastewater is eliminated biologically (bio-P). The rest can be
(NH -N). During biological wastewater treatment, organic N is
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con- verted to NH -N by the bacteria in the activated sludge. This removed by chemico-physical phosphate precipitation.
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60 Water Today - The Magazine March 2020