UPGRADING A LOW-COST PHYSICOCHEMICAL WASTE-WATER TREATMENT-PLANT TO SOLVE OPERATIONAL PROBLEMS

A comprehensive programme of jar tests utilizing lime to treat municip al wastewater in northeast Brazil showed that a supernatant could be a chieved that attained the WHO (1989) guidelines for effluent reuse for unrestricted irrigation (less than or equal to 1000 faecal coliforms per 100 mi; less than or equal to 1 intestinal nematode egg per litre) . In the light of these findings a pilot-plant was built to investigat e the continuous treatment of wastewater with lime for effluent reuse in irrigation. Bearing in mind the problems often encountered in less- developed countries with the operation and maintenance of complex wast ewater treatment systems, it was decided to base the lime treatment te chnology on that used successfully throughout the world (in both indus trialised and non-industrialised countries) to treat potable water; na mely non-mechanised coagulation and flocculation, and settlement in a horizontal-flow sedimentation tank. Although the prototype pilot-plant achieved a final effluent that attained the WHO (1989) guidelines for unrestricted irrigation, a number of operational and maintenance prob lems were encountered. Effluent quality data has been reported previou sly. This paper summarises the data and describes the practical proble ms associated with the operation and maintenance of the lime treatment plant, and the upgrading techniques used to overcome these difficulti es. The prototype plant consisted of a rapid-mix basin in which raw mu nicipal wastewater was combined with lime slurry at a ratio that achie ved a pH of greater than 11.0 after mixing; a sedimentation tank, for solid-liquid separation; and a recarbonation basin for tertiary treatm ent. Constant monitoring of the lime-wastewater mix pH, using a pH con troller to regulate the flow of lime slurry, proved difficult. Frequen t cleaning of pipework was needed to avoid clogging by the lime slurry . In a second phase of the research work, the pilot-plant was upgraded to tackle these and other operational and maintenance problems. Plant geometry was improved to prevent the sedimentation of lime solids wit hin the delivery pipework. All pipework carrying lime slurry was set u p in duplicate so that plant operation could continue during cleaning operations. An improved pH control system, which linked lime slurry fl ow to the pH of the lime-wastewater mix, was also installed. The desig n of the sedimentation tank was altered to aid the removal of sludge a nd the tank inlet and outlet were upgraded in order to minimise short- circuiting. Plant operation and maintenance were simplified by the des ign improvements and the frequency of mechanical breakdown was reduced . The pH control system was difficult to operate effectively because t he pH probe became rapidly fouled by lime solids. However, gel-filled, low sodium-error pH electrodes calibrated in pH 11.0 buffer proved si gnificantly more robust in the aggressive high-lime environment than d id conventional combined glass electrodes. Options for further improvi ng the design of a lime treatment plant are considered.