treatability tests

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oxygen consumption measurements: respirometry

As previously discussed (fundamental biological engineering processes applicable to water treatment), measurements used to monitor oxygen consumption in an aerobic biomass constitute one of the preferred methods for ascertaining either the biodegradability/toxicity of an effluent’s components ( BOD ) or the condition of the biomass and its potential in relation to a given effluent ( BOD or specific pollution components removal kinetics).

manometric measurement: Warburg respirometry

The Warburg respirometer is not so much an inspection machine but rather a machine used to study the respiratory action of manometrically activated sludge, using a small volume sample (a few mL) depending on the oxygen concentration.

The respirometer can be used to determine the respiratory coefficients a’, b’ of an activated sludge as illustrated by figure 14.

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Figure 14. Principle of the Warburg determination of respiratory coefficients

Very delicate to operate, Warburg respirometers are, in fact, virtually not used any more. These respirometers have now been replaced by systems such as the one shown in figure 15 and used to carry out a direct and continuous measurement of the oxygen consumption rate in a closed reactor.

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Figure 15. RO2 measurement by calculating the rate at which the O2 concentration declines in a closed cell

In measurement cell 10 (figure 15), in the absence of any incoming air, the dissolved oxygen concentration decreases linearly and in proportion to micro-organism activity (figure 16).

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Figure 16. Changes in O2 concentration

In measurement cell 10 (figure 15), in the absence of any incoming air, the dissolved oxygen concentration decreases linearly and in proportion to micro-organism activity (figure 16).

Measuring the reactor's biomass concentration enables us to establish the amount of oxygen consumed at the temperature set by the thermostat; g O2 per g VM per h.

This respirometer can be used to:

  • measure an effluent’s toxicity (intermittently in order to establish a toxicity threshold or in-line when acting as an alarm to detect an abnormally low RO2):
  • establish an effluent’s biodegradability (rapidly degradable fraction, slowly degradable fraction, refractory fraction);
  • measure respiratory coefficients.

nitrification test

An effluent seeded with nitrifying sludge or specific strains is aerated and agitated in a flask. The NO3, NO2 and NH4 forms of nitrogen are checked at regular intervals. The curves showing the development of nitrogen species are used to assess treatability and oxidation kinetics. These tests are mainly applied to IWW when the presence of an inhibitor is suspected.

denitrification test

This guideline test is used to evaluate denitrification kinetics in either an IWW or in a nitrified UWW with or without the addition of assimilable carbon (e.g. methanol).

Figure 17 allows us to see how the test is applied. The flasks used are blanked off with a septum that allows a syringe to be used to take samples and to make injections without altering the medium’s anoxia (preliminary nitrogen sweep).

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Figure 17. Principle of a denitrification test

test bench

aerobic medium

Continuous or batch (small SBR ) operation reactors are used; excess air is usually injected into these reactors. Global intake and discharge COD and BOD parameters are monitored on the basis of time. Continuous checks can be carried out in the reactor on pH, redox potential, oxygen, TOC …. and also on suspended solids and VM.

Figure 18 represents laboratory apparatus that forms a veritable small wastewater treatment plant with aeration and clarification zones. After aeration, activated sludge is sent into the bottom section of a clarification cylinder and recycled by an «emulsifier» after being separated from the treated water. This is a complete mixing unit; this simplifies the mathematical equations that can be written using the operating parameters observed

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Figure 18. Combined apparatus used as a laboratory analysis pilot

The emulsifier can be replaced by a peristaltic pump in order to precisely regulate the recycling flow rate and to separate aeration from return sludge pumping.

In order to acclimatize activated sludge to a problematic IWW, we usually start with sludge taken from an urban wastewater treatment plant and the pilot is supplied with a mixture of urban water and water to be tested. Within approximately ten days, the proportion of IWW is gradually increased at the rate of one or two increments per day. If we reach 100% IWW, we can assume that this water is suitable for treatment; otherwise, we can establish in what proportion this IWW has to be diluted with UWW in order to ensure that it persists as a viable substrate for micro-organisms. Obviously, this test has to be carried out on a medium that has been balanced, whether naturally or artificially, in terms of assimilable carbon, nitrogen and phosphorus.

anaerobic medium

Intermittent fermentation is carried out in small reactors (photo 6), seeded using sludge from urban sludge digesters having a wide spectrum population. Initial operations are carried out under nitrogen bubble aeration before the flask is sealed hermetically. The baths are stirred and kept in an oven at 37°C. Gas and effluent samples are collected at regular intervals to check on methane formation and on COD reduction.

We can also use attached growth, mixed growth or granular sludge bed reactors (figure 19) that are continuously fed.

Checks on gas output and composition, on pH, temperature, VFA, TAC COD … are carried out on the supply and on the treated water.

This enables us to determine the biodegradable fraction of the effluent, to obtain information on degradation kinetics and even on inhibition phenomena and on effluent toxicity. (Comment: a 3-4 month study is often required in view of the length of the sludge latency and acclimatisation phases).

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Photo 6. Laboratory fermentation unit
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Figure 19. Laboratory methane fermentation unit (for submersion in a thermostatically controlled bath)