Efficient Wastewater Treatment
The field for analytical and monitoring Equipment

By Siegfried Forster
July 2000

The Author is a specialist in water analysis with Wissenschaftlich-Technische Werkstätten (WTW) in Weilheim, Germany.

Clean drinking water is an essential need of mankind. Often surface water like rivers or lakes is used as drinking water source. To prevent these resources from pollution the effective treatment of waste water is a must.
For example in China 75% of discharged waste water is coming from pulp and paper, textile and dyeing industries. To run a waste water treatment plant in the most efficient way certain measurement technologies have to be available. In respect to size of the plant and individual efficiency objective 3 different types of measurement parameters have to be distinguished.

  1. Parameters that are essential to ensure the cleaning process on a waste water treatment plant like pH and DO
  2. Parameters that shows the efficiency of the plant like BOD and COD
  3. Parameters helping to optimise the plant like DO, ammonium, nitrate, nitrite and phosphorous

Parameters that are essential to run a waste water treatment plant – pH-value and DO-concentration

To run a waste water treatment plant at all the pH-value as well as the dissolved oxygen (DO) content have to be monitored. A stream of inlet water with a pH-value outside of a range from 6.0 to 9.0 will cease the activity the micro-organism needed for the cleaning process. Besides that a pH-value below 6.5 will destroy the concrete of the facilities.
During the cleaning process a sufficient amount of dissolved oxygen for growth and metabolism of micro-organism has to be ensured. A DO concentration of 1-2 mg/l is sufficient in a aeration tank. Higher DO-content will not necessarily increase the cleaning efficiency. Blowing air into the aeration tank should be adjusted to that DO-concentration of 2 mg/L, more is just wasted energy.
For every plant an optimum DO-concentration depending on the type of micro-organism and waste water matrix can be evaluated by changing the DO-concentration and control the efficiency parameters BOD and COD.
The basic instrument to measure these parameters is a "2 in 1" hand held meter, the MultiLine P3 pH/Oxi from WTW. This handy, easy to use, robust and waterproof instruments performs with low costs the most important parameters for wastewater monitoring. The meter comes complete with sensors, calibration- and maintenance solutions for pH, dissolved oxygen and temperature measurement in a rugged field carrying case.
For sure fixed installed measurement systems (pH-value at the inlet, DO in the aeration tank) that are able to give alarms, start or stop pumps are the most comfortable way. Especially monitoring of DO-concentration to run the plant on their optimum point of efficiency On-Line monitoring is necessary. WTW offers also these type of instrumentation, e.a. the Ecoline-transmitter series. With its large choice of options and accessories these fixed installed transmitters and probes can be optimised for every plant.

Parameters indicating the efficiency of the plant – BOD and COD

These 2 parameters are the most important ones to determine the pollution of waste water. Knowing these 2 values at the inlet and the effluence of the plant makes it easy to judge on the efficiency of the plant.

The Biological Oxygen Demand (BOD5)
The BOD5 is an indirect measurement of carbon compounds by direct measuring the amount of oxygen consumed by the micro-organism at a temperature of 20°C within 5 days. Knowing the difference of BOD5 levels from inlet and effluence of the wastewater plant makes calculation of the cleaning efficiency easy. Common values of BOD5 are: 400-1000 mg/l at the inlet of the treatment plant and below 50 mg/l at the effluence of the plant.

2 different methods for BOD determination are commonly used.

The Chemical Oxygen Demand (COD)
The COD value indicates the oxygen concentration needed to oxidise all carbon compounds in the sample. The COD measurement is based on a thermal reaction of the sample with chemicals during a heating period of 2 hours at 148°C. This reaction will cause a change in colour which can be measured with a photometer.
Typical values of COD are 500-1000 mg/l at the inlet of the plant and below 75 mg/l at the outlet of the plant.
The PhotoLab S6 photometer and the belonging prefilled cuvette tests offers easy to use COD-determination. Just filling a certain amount of sample into the cuvette, heat in in the thermal block CR 2010 for 2 hours and stich the cuvette after cooling down into the photometer. The PhotoLab S6 will automatically recognize the type and measurement range of the used test by reading the barcode on the cuvette and within seconds the COD-value will be displayed on the large display.

Parameters helping to optimize the plant

Total nitrogen resp. the single parameters ammonium, nitrate and nitrite are important parameters to qualify wastewater. High contents of ammonium coming out the plant into a river or lake will cause a significant reduction of natural DO content by forming nitrate and nitrite. Nitrate is a nutrient which will fertilise the rivers and lakes. Nitrite is poisonous for fish and micro-organism. The value of total N at the outlet of a waste water plant or in rivers and lakes should be below 18 mg/l. For control of a nitrification/denitrification-process in a waste water treatment plant ammonia measurement is a must.

Phosphate in wastewater will fertilise the rivers and lakes. Due to this eutrophication process algae and seaweed get highly abundant causing massive impact on the ecological balance of the ecosystems.

For measurement of nitrogen and phosphorous again the PhotoLab S6 with the belonging cuvette tests are the best choice.

As described a very limited number of WTW instruments like MultiLine P3 , OxiTop and PhotoLab S6 incl. some accessories will already allow to measure the most important wastewater parameters.
Enlargement of the plants and optimising measurement technologies is later on possible with a step by step replacement of the intermittent measurements through continuous working analyzing equipment starting at the pH-measurement at the inlet to ammonia and phosphorus analytics at the effluent.


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