Optimal Nutrient Ratios for Wastewater Treatment

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Optimal Nutrient Ratios for Wastewater Treatment

To be able to comply with the legal requirements on treated wastewater, plant operators must control the treatment process carefully so they can intervene promptly to prevent limit values from being exceeded. Besides chemical and physical methods, wastewater treatment is essentially based on biological treatment by microorganisms in activated sludge. Knowledge of the nutrient requirements and the composition of the activated sludge are therefore needed if the plant is to operate at maximum efficiency. The causes and effects of unfavorable nutrient ratios, and the measures taken to deal with them, are described in this report.

Nutrients in activated Sludge

A balanced nutrient ratio is essential if the microorganisms are to function at maximum efficiency. The most important of these nutrients are carbon, nitrogen and phosphorus.

Carbon is the principal component of the organic substances found in wastewater.

It is biodegraded by the microorganisms in activated sludge under anaerobic conditions (bio-P), in an anoxic environment (denitrification zone), and in the aerated part of the biological stage (nitrification zone). The microorganisms use the carbon compounds to build their cell structures and to generate energy.

Carbon compounds are determined as chemical oxygen demand (COD), biological oxygen demand (BOD) or total organic carbon (TOC)

In the inflow of wastewater treatment plants, nitrogen is present in organically bonded form (organic N) and as ammonium nitrogen (NH4-N). During biological wastewater treatment, organic N is converted to NH4-N by the bacteria in the activated sludge. This NH4-N and the NH4-N from the inflow are converted to nitrite, which in turn is converted into nitrate (nitrification).

The nitrogen compounds that are not biodegraded in the activated sludge are converted under anoxic conditions (absence of dissolved O2) to elementary nitrogen (denitrification). This escapes into the atmosphere as N2. Nitrogen compounds are determined as NH4-N, NO2-N, NO3-N, and TN (total nitrogen, which is important for balancing and outflow checks).

The P load in the inflow of a wastewater treatment plant is made up of orthophosphate-phosphorus (PO4-P), polyphosphates, and organic phosphorus compounds. Together, they give the sum parameter ‘total phosphorus’ (Ptot).

Laboratory analysis and process measurement technology support compliance with the limit values.

During biological wastewater treatment, polyphosphates and organically bonded phosphorus are converted to orthophosphate.

The P demand of the organisms is due to the special role of phosphorus in their energy metabolism. P is needed to form the cell membrane and DNA.

Some of the phosphorus in wastewater is eliminated biologically (bio-P). The rest can be removed by chemicophysical phosphate precipitation. Phosphorus compounds are determined as ortho-PO4-P (control of precipitation) and as Ptot (balancing, outflow monitoring).

Trace elements
Other trace elements needed to build cells (potassium, magnesium, manganese, iron, copper, zinc and nickel, and vitamins and growth factors) are usually present in municipal wastewater, or the microorganisms in the activated sludge provide them themselves.

Septic domestic wastewater and some industrial wastewater contain reduced sulfur compounds (hydrogen sulfide, sulfides and thiosulfates). Sulfur is an indispensable component of proteins. In wastewater treatment plants, reduced sulfur compounds are not only oxidized chemically to sulfate but are also oxidized by some bacteria to form sulfur and, since this process generates energy, are stored inside cells as food reserves. High concentrations of reduced sulfur compounds in wastewater can, however, cause a number of problems.

C:N:P ratio (BOD:TN:Ptot)
The content of the individual nutrients in wastewater should correspond to the needs of the bacteria in the activated sludge, and there should be a balanced relationship between C, N and P. This is crucial to the effectiveness of the biodegradation processes. During aerobic wastewater treatment, the C:N:P ratio should be in the range between 100:10:1 and 100:5:1.

Favorable and unfavorable nutrient ratios

All sorts of industrial plants; regional differences in eating habits (disposal of different kitchen wastes through the drains), and the nature of the soil and drinking water cause wastewater to vary widely in its composition. Experience has shown that the C:N:P ratio in municipal wastewater is about 100:20:5.

The excess N and P compounds can usually be eliminated from the wastewater without any great difficulty using modern methods.

If the wastewater in the inflow to the biological stage is deficient in one of the main nutrients, a wide range of problems may occur.

For efficient denitrification, a certain proportion of readily biodegradable C compounds must be present. After municipal wastewater has passed through the primary settling tank, it has a BOD:N ratio of 100:25 (= 5). If the ratio falls below 100:40 (= 2.5), the efficiency of the denitrification process is impaired, resulting in higher nitrate values in the outflow. If bypassing the primary treatment and increasing the denitrification volume fail to bring about any improvement, the addition of a readily degradable substrate (external source of carbon) should be considered.

Carbon sources for nutrient balancing include:

  • Internal C = hydrolyzed or acidified primary sludge
  • External C = industrial residues (from breweries, dairies, sugar industry) and industrial products (methanol, ethanol, acetic acid)

COD:BOD ratio
The ratio of these two sum parameters is a measure of the biodegradability of the wastewater pollution load. If the COD:BOD ratio does not exceed 2:1, the biodegradability is said to be good. Higher values indicate the presence of poorly biodegradable substances.

See an example of testing calculations, typical measurement locations, and other information in the brochure.

For more information on wastewater testing and equipment, visit www.hach.com.

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