The fouling rate, measured as the slope of transmembrane pressure against filtration time, has been used in many works as a fouling quantification parameter in systems operated under constant permeate flux. Experimentally, it has been found that rf depends exponentially on permeate flux (Figure 12). Therefore, a threshold flux value may be identified (32 l h-1 m-2) above which the fouling increases at an unacceptable rate.

4.3 Physico-chemical and microbiological quality of the permeate

The physical and chemical quality of the permeate was assessed by the analysis of turbidity, COD and nitrogen compounds.

The permeate had an average turbidity value of 0.59 NTU, indicating a total retention of suspended solids and macro-colloidal matter. In addition, the low turbidity of the permeate registered during the whole experimental period showed that the membrane maintained its integrity.

Fig. 12. Fouling rate against permeate flux.

The organic matter content was determined by measuring the COD in feed wastewater, in the permeate and in the liquid phase of the suspension. Soluble COD (CODS) was obtained by filtering through a filter paper of 0.45 pm pore diameter. Figure 13 shows the COD of feedwater (COD feed), the soluble COD of feedwater (CODs feed), the COD of the permeate (CODp) and soluble COD of the liquid phase (CODs reactor) versus operating time. Typical fluctuations of feed wastewater can be seem in a real treatment plant. These oscillations lessened considerably in the permeate and in the liquid phase.

Fig. 13. COD evolution with operation time.

Fig. 14. Evolution of the nitrogen compounds with operation time.

As it is shown in Figure 13, there is a significant difference between the total and soluble COD of feed due to the presence of suspended solids. It was estimated that approximately 68% of the COD of the feed is in a particulate form. If the soluble COD of feed is compared with the soluble COD of the CODs liquid phase (CODs reactor) a removal efficiency close to 86% can be obtained, mainly due to biological degradation and only 6% is due to the membrane separation process. It should be noted that the BOD5 was not analyzed because, through frequent and trustworthy analysis of the same water, the BOD5/COD ratio was

confirmed to be approximately constant and equal to 0.75, so the COD analysis may be considered sufficient to determine the biodegradation produced.

Also, the evolution of the ammonium nitrogen concentration in feed wastewater (N-NH4 feed) and the nitrogen compounds of the permeate ((N-NH4+)p, (N-NO2-)p, (N-NO3-)p) were measured during the experimental period (Figure 14). As can be seen, the concentrations of nitrogen-nitrate in the permeate (N-NO3-)p were in the range of 15-45 mg/l, while nitrite and ammonia were completely removed. This is interpreted as a total oxidation of ammonium to nitrate.

As shown in Table 5, no bacterial contamination indicators, bacterial pathogens or parasites were detected in the permeate. This is attributed to the ultrafiltration membrane which has a pore diameter smaller than the size of bacteria and parasitic microorganisms, so that the membrane is an effective barrier. However, Table 5 shows the presence of viral indicators. Here, results indicate a great degree of removal (99.8% and 95.3% for somatic coliphages and F-RNA bacteriophages, respectively).

	Feed wastewater	Permeate (N = 3)
Bacteriological indicators
Fecal coliform[1]	7.7106	absence
Escherichia Coli[1]	7.3 106	absence
Enterococci[1]	3.6 106	absence
Clostridium perfringens[1	1.1106	absence
Indicators of pathogenic contamination
Pseudomonas aeruginosa[1]	absence	absence
Salmonella sp. [1]	absence	absence
Viral indicators
Somatic coliphages[2]	3.2 106	4.3 103 ± 1.6-103
F-RNA bacteriophages[2]	2.3 105	1.1104 ± 1.6104
Parasites
Giardia lamblia [3]	absence	absence
Cryptosporidium sp. [3]	absence	absence
M CFU/100ml; И PFU/100ml; И No/100 ml. N= Number of samples

Table 5. Feed wastewater and permeate microbial results.

Permeate microbial results proved that MBR systems are able to produce permeate of high microbial quality to be used in several applications such as land irrigation, agricultural activities etc., in accordance with local standards.

5. Conclusions

(>95%) and constant COD removal efficiency (80-98%) was achieved, regardless of the influent fluctuations. Microbial analysis of permeate showed the absence of bacterial indicators of contamination and parasitical microorganisms. At the same time, the membrane presented over 98% efficiency in the elimination of viral indicators.

Particularly interesting is the possibility of operating at maintenance energy level of the biomass, which significantly reduces sludge production. At these maintenance conditions, a minimal value for the carbon substrate utilization rate (0.07-0.1 kg COD kg-1 MLVSS d-1) was found and the system was operated successfully at permeate flux between 30 and 32 l h-1m-2 and low physical cleaning frequency. As a result of carbon substrate limited conditions, EPSs were minimized and higher organisms appeared.

Biomass development at maintenance conditions can be well described by the kinetic model based on Pirt’s equation.

Although there are many practical experiences for MBR design and operation, there are still some aspects that are not completely understood. Without any doubt, the most cited is membrane fouling. The complexity of this phenomenon is linked to the presence of particles and macromolecules with very different sizes and the biological nature of the microbial suspensions which results in a very heterogenic system. Meanwhile, the dynamic behaviour of the filtration process adds a particular complication to fouling mechanisms. Therefore, further investigation is required so as to ascertain which component in the suspension is the primary cause of membrane fouling.

6. Acknowledgements

This work has been funded by the N. R.C. (MEC project CTM2006-12226). The authors also want to express their gratitude to the MEC for a doctoral scholarship, to GE ZENON, to CANARAGUA and to BALTEN for their support and finally to the Water Analysis Laboratory of the ULL Chemical Engineering Department for analytical advice.

7. Nomenclature

CAS Conventional activated sludge process

COD Chemical oxygen demand, mg O2 /l

EPS Extracellular polymeric substance

F/M Feed to microorganisms ratio, kg COD/kg MLSS d

HRT Hydraulic retention time, h

iMBR Immersed membrane bioreactor

J Permeate flux, l/h m2

MLSS Mixed liquor total suspended solids, mg/l

MLVSS Mixed liquor volatile suspended solids, mg/l

NH4-N Ammonium nitrogen concentration, mg/l

NO2-N Nitrite nitrogen concentration, mg/l

NO3-N Nitrate nitrogen concentration, mg/l

SADm Specific membrane aeration demand, Nm3/h m2

SOURe Specific oxygen uptake rate in endogenous conditions, kg O2/kg MLVSS d

SRT Sludge retention time, days

TMP Transmembrane pressure

U Utilisation rate, kg COD/kg MLVSS d