Avaliação das concentrações de nutrientes (N e P) na lagoa anaeróbia profunda da pedreira nº 7 do polo de tratamento de esgotos do baixo Paraíba.

Abstract

The aim of this work was to evaluate the behaviour of nutrient species (N and P) in a full scale sewage treatment system, made up of a deep (8 m) anaerobic pond developed from an inactivated quarry of limestone exploitation, situated in the Basin of Paraiba River in the City of Joao Pessoa (7° 10" South; 34° 49" West), Paraiba state, Northeast Brazil. Nutrient loadings from the anaerobic pond discharged into the estuary of Paraiba River were also estimated. Monitoring of both influent raw sewage and the effluent of the anaerobic pond was carried out from March to November 2005 with samples collected weekly. The variables pH, temperature, BOD5, COD, total kjeldahl nitrogen, ammoniacal nitrogen, total phosphorus and soluble orthophosphate were analyzed according with the recommendations of APHA et al. (1998). With the objective of evaluating daily fluctuations of the monitored variables two studies (profiles) were carried out through the daily cycle with analysis of samples of rawsewage and pond effluent collected at 6 and 11 a.m. and 5 and 10 p.m. Based on flow-rate data collected from 27t h April to 13* June 2005, a daily average flow of 25890 mVd and a mean hydraulic retention time of 3.27 days were estimated. The volumetric organic load was of 82.5 gBODs/m3.d, being verified that the deep anaerobic pond did not show a typical performance of conventional anaerobic ponds, as described in literature. In fact, its efficiency in terms of organic matter removal was less than that observed in a study carried out two years ago. Removals of the nutrient species were very low. Total kjeldahl nitrogen removal efficiency (7.04 % ) was mainly due to organic nitrogen removal in the reactor being observed an increase in the ammoniacal nitrogen The removal of total phosphorus was only 4.91% and soluble orthophosphate increased 18.4% due to phosphorus transformations and orthophosphate diffused from the sludge layer. Nutrient concentrations in the effluent of the anaerobic pond were well above the standards for discharges into receiving water bodies being recommended the system to be upgraded by incorporating additional treatment units which could be developed from inactivated quarries of limestone exploitation nearby.