UNCOUPLED ENERGY METABOLISM FOR SLUDGE REDUCTION IN THE ACTIVATED SLUDGE PROCESS - Biochemical Engineering - page 156

Zheng et al. [4] studied the feasibility of 2,4,6-trichlorophenol (TCP) and malonic acid
(MA) as metabolic uncouplers for sludge generation reduction in sequence batch reactors
(SBRs) for treating organic wastewater for a long period. Results showed that TCP at the
concentrations of 2 mg L -1 and 10 mg L -1 could reduce sludge production by about 47% and
30%, respectively, while the effluent COD removal efficiency and sludge settleability were
not significantly affected. Compared to MA, TCP was found to be a better metabolic
uncoupler for sludge reduction for a long period of operation.
Chemical uncoupling may be the simplest method for biomass reduction as it just
requires the addition of certain amount of uncouplers and no other plant changes or extra
steps are needed. Actually, industrial uncoupling application in two full-scale activated sludge
plants in Phoenix Arizona was reported by Okey and Stensel [77]. It appears from the
literature that addition of chemical uncouplers to biological wastewater treatment systems can
significantly reduce sludge production, and can be regarded as a promising technique for
reduction of excess activated sludge generation in CAS process. However, long-term
bioacclimation has been observed and can eventually outweigh the advantageous effects of
uncoupler addition. Application of chemical uncoupler for sludge reduction may cause
reduction of COD removal and even worsen the settleability and dewaterability of activated
sludge. It should also be pointed out that most chemical uncouplers are xenobiotics and
potentially harmful to human health and the environment, as well as to the microbial cells.
Optimization and correct dosing are important to avoid inhibition to microorganisms instead
of uncoupling. Thus, thorough consideration and careful plant management should be taken in
practice, and further research is needed for the environmental impact of chemical uncouplers
in long term [5]. It was also found that addition of metabolic uncouplers would result in
higher consumption of dissolved oxygen [78]. In addition, many factors, such as culture
medium, pH, temperature, the affinity of microbes to different types of metabolic uncouplers
and physiological state and viability of microorganism, may have significant effects on the
efficiency of sludge reduction by metabolic uncouplers [1].

Table 4. Eeffects of various uncouplers on sludge reduction and effluent quality

Compound Dosing & effects Reference

0.4 mg L -1 : shreshold; 0.8 mg L -1 : 40% sludge reduction;
0.5-1.0 mg L -1 : no effect on substrate removal efficiency.

[73]

1.0 mg L -1 : 50% sludge reduction; 2.0-3.6 mg L -1 dosing every
other day for 40 days: no effect on effluent nutrient
concentration.

[75]

TCS

40 mg d -1 : 30% sludge reduction; 60-day continuous operation:
increase effluent nitrogen concentration.

[23]

DNP ＜ 10 mg L -1 : decrease sludge production significant;
DNP￿10 mg L -1 : inhibit the relative specific COD removal rate.

[71]

2,4-DNP

2 mg L -1 : 47% sludge reduction, with slight decrease of COD
removal efficiency and sludge settleability.

[79]

TCP

DCP ＜ 5 mg L -1 : didn't change the diameter distribution of
sludge flocs; 1-20 mg L -1 : slightly reduce SOUR and sludge
yield.

[80]

2,4-DCP

100 mg L -1 : 62% sludge reduction with increase in SOUR PNP [81]