The treatment of municipal and industrial wastewater produces sludge, which either must be used for beneficial purposes or requires disposal (Malina, 1993). Sludge can be considered as an aqueous suspension of particles together with some compound salts (Gillium and Lecomte, 1999). Secondary sludge consists predominantly of excess biomass produced during the biological process (Ramalho, 1983). About half of the incoming organic pollution load is converted into secondary sludge, which contains 0.5 to 2 % solids (Malina, 1993; Winkler, 1993). Since a large number of the enteric bacteria and viral pathogens presented in untreated sewage become associated with wastewater solids, many are not completely removed during sewage treatment processes and are merely transferred to wastewater sludge (Farrah and Bitton, 1983). The latter is further treated to convert it into a small volume of stable material with high solid content and low level of heavy metals, pathogens and persistent organic substances (Winkler, 1993). Anaerobic digestion processes are widely recognised as particularly suitable for highly polluted wastewater treatment and for the stabilisation of primary and secondary sludges (Genovesi et al. 1999).

There are few reports on anaerobic digestion of secondary sewage sludge yielded by up flow anaerobic sludge blanket reactors (UASB), because it is considered that sludge has achieved stabilisation inside the UASB reactors. However, the optimisation of UASB reactors allows sewage treatment at a low hydraulic retention time; therefore, the produced sludge may not achieve stabilisation, and it may even be necessary to include a supplemental stabilisation process out of the reactor (Fernandes, 1998).

Sewage sludge disposal is a serious worldwide problem. Because of increased environmental awareness and stringent environmental standards governing the disposal of sewage sludge (set by different environmental protection agencies), its utilisation in agricultural production has been gaining increasing interest and attention in recent years. It offers economic and nutrient recycling advantages over the traditional disposal options, such as incineration for dry sewage and sea disposal (Stone et al. 1998). Nevertheless, potential risks derived from the accumulation of heavy metals and organic compounds, as well as pathogen contamination, must be taken into consideration.

A nine-month sampling program was carried out in a RALF (anaerobic reactor of fluidised sludge) reactor, a variation of UASB reactor, located in a local urban wastewater treatment plant. The purpose of this program was to investigate the seasonal fluctuations on the physicochemical characteristics and on the levels of the pollution indicator bacteria in excess sludge yielded by the RALF reactor, as well as to investigate the sanitary efficiency of low rate anaerobic digestion as a supplemental stabilisation process. Thus, a total of three independent batch experiments, carried out with an anaerobic digester at an operation time of 60 days, was evaluated through physicochemical, microbiologic and parasitologic analysis. The indicators chosen included total culturable, the total and fecal coliform groups, Pseudomonas aeruginosa and fecal streptococci. Parasitological analysis was also performed on multiple sludge samples by determination of protozoa and helminth eggs. Furthermore, the viability of utilisation of sewage sludge in agricultural lands was analysed considering the environmental risks.

Experiments were carried out with sewage sludge, without any previous thickening, obtained from one of the local municipal wastewater treatment plants (Maringá-Paraná, Brasil). The wastewater treatment system consists of a screening/grit device for the removal of coarse solids followed by a RALF reactor. In the treatment sequence, in the wastewater treatment plant, the sewage sludge produced is spread on drying beds. These beds contain gravel for rapid drainage and are exposed to outdoor conditions. In each experiment, the sewage sludge was collected approximately every three to four months during the discharge of excess sludge from the RALF reactor, before draining and drying in the drying beds.

Laboratory experiments were carried out in a 70-liter cylindrical anaerobic digester with a fixed cover, manufactured in polyvinyl chloride and without mixing and temperature control. A series of three independent batch experiments was performed for an operation time of 60 days at room temperature.

In general, the excess sludge discharged by the RALF presented seasonal characteristics because of variations in incoming wastewater and variations in the performance of the treatment process. The influent total solids concentration ranged from 5 to 8%, of which an average of 59% was organic matter. Influent pH, TSS, VSS, protein and COD concentration in the influent sludge varied, respectively, from 7.2 to 7.5, 37.9 to 56.3 g L^-1, 23.7 to 29.2 g L^-1, 9.8 to 18.7 g L^-1 and 32.9 to 65.7 g L^-1. Metal concentrations of influent sludge also presented variations, especially for chromium and iron that ranged from 0.04 to 0.1 g Kg^-1 and from 39.0 to 63.3 g Kg^-1, respectively.

The operation of anaerobic digestion was stable, with no noticeable scum or foaming problems. The batch experiments were operated at an average room temperature throughout the anaerobic digestion of 23.1, 24.6 and 25.2ºC (daily average temperature basis), respectively. Because of technical problems, the VSS content was not measured during the first experiment. Removal efficiency of VSS reached 43 and 20% in sludge and 98 and 81% in the supernatant of the second and third experiments, respectively. The efficiency of protein removal in sludge reached 25, 28 and 27% in each experiment. COD reduction reached 29, 21, and 45% in the sludge and 95, 85, and 82% in the supernatant in each experiment, respectively. These results show that organic matter both in the sludge and in the supernatant was digested, sensibly decreasing the pollutant load.

The microbial indicators were surveyed by sampling the sludge throughout the digester operation and counting the number of bacteria in the sampled sludge. Counted bacteria included the total culturable, the total and fecal coliform groups, Pseudomonas aeruginosa and fecal streptococci. The percentage removal of the indicator bacteria was higher for fecal streptococci (99.9%) than for coliform bacteria (96.3%), which in turn was higher than for P. aeruginosa (95.6%). In addition, P. aeruginosa isolated from sludge presented a level of antibiotic resistance lower than the ones reported in literature for clinical isolates, thus representing no risk to bacterial biota in nature.

Parasitological analysis was also performed on multiple sludge samples by determination of protozoa and helminth eggs. Protozoa (Eimeria and Entamoeba), helminth eggs (Ascaris, Trichuris, Toxocara, Hymenolepis) and mites were detected in the influent sludge, and particularly among the helminth eggs, only Trichuris was detected in the effluent sludge.

According to the European Directive 86/278/EEC (Council of the European Communities, 1986), the heavy metal content is not a limiting factor in the use of the sludge as a soil conditioner. The sludge has fertilizing potential, however, the phosphorus and potassium content of sludge may be too low to satisfy specific plant uptake requirements in some land application systems.

In Paraná, Brazil, the state sanitation company classifies the sludge according to the American CFR Part 503 regulation and specifies class A sludge (10³/g dry wt basis) for urban sludge spreading on agricultural land (Andreoli et al. 1999). The presented data show that the fecal coliforms in influent sludge must be reduced to below detectable levels to reach the land-use criteria. Therefore, the effluent sludge can fulfil the criteria for category A sludge, especially if the entire process, such as the anaerobically digested sludge analyses after being air-dried on drying beds, has been considered.

Analysis of anaerobically digested sludge after being air-dried on drying beds would be helpful in the viability of land application of treated sludge. Based on this data, criteria for estimating pollution mobilisation capacity and behaviour in the environment could be developed. However, considerable progress has been achieved in this direction by providing information concerning the efficiency of anaerobic digestion of secondary sludge in decreasing the bacteria population and the determination of possible pollution levels for land application of the treated sludge.

ANDREOLI, C.V.; LARA, A.I. and FERNANDES, F. Reciclagem de Biossólidos: Transformando Problemas em Soluções. SANEPAR, FINEP, Curitiba, Brazil, 1999. 300 p.

COUNCIL OF THE EUROPEAN COMMUNITIES. Directive concerning sludge amendments. Official Journal L181, 04/07/1986, p. 6-12.

FARRAH, S.R. and BITTON, G. Bacterial survival and association with sludge flocs during aerobic and anaerobic wastewater sludge under laboratory conditions. Applied and Environmental Microbiology, 1983, vol. 45, no. 1, p. 174-181.

FERNANDES, F. Produção e processamento de biossólidos: estabilização e higienização. In: Proceedings of the I Seminário sobre Gerenciamento de Biossólidos do Mercosul. (1º, 1^st - 4^th December, 1998, Curitiba, Brazil). ABES, SANEPAR, 1998. p. 47-50.

GENOVESI, A.; HARMAND, J. and STEYER, J.P. Fuzzy fault detection and isolation of an anaerobic digestion pilot-plant. In: Proceedings of the Congress of Chemical Engineering.  (2º, 5^th  - 7^th October, 1999, ECCE 2, Montpellier, France). 8 p.

GILLIUM, C. and LECOMTE, D. Freeze-thaw conditioning of sludges. In: Proceedings of the Congress of Chemical Engineering. (2º, 5^th  - 7^th October, 1999, ECCE 2, Montpellier, France). 9 p.

MALINA, J.F. Sludge handling, treatment and disposal - an overview. In: Proceedings of the II Seminar of Technology Transfer - Sludge Treatment and Final Disposal. (2º, 13^th -  16^th December, 1993, Rio de Janeiro, Brazil). ABES, WEF, 1993. p. 1-17.

RAMALHO, R.S. Introduction to wastewater treatment processes. 2^nd ed. Academic Press, California, 1983. 409 p. ISBN: 0-12-576560-6.

STONE, R.J.; EKWUE, E.I and CLARKE, R.O. Engineering properties of sewage sludge in Trinidad. Journal of Agricultural Engineering Research, 1998, vol. 70, p. 221-230.

WINKLER, M. Sewage sludge treatments. Chemistry & Industry, April 1993, p. 237-240.

Note: Electronic Journal of Biotechnology is not responsible if on-line references cited on manuscripts are not available any more after the date of publication.