Upgrading a low-cost physicochemical wastewater treatment plant to solve operational problems
Authors: Taylor Huw D. , Gambrill Martin P. , Mara D. Duncan , Silva Salamão A. 
Published: May 2018

A comprehensive programme of jar tests utilizing lime to treat municipal wastewater in northeast Brazil showed that a supernatant could be achieved that attained the WHO (1989) guidelines for effluent reuse for unrestricted irrigation (≤ 1000 faecal coliforms per 100 ml; ≤ 1 intestinal nematode egg per litre). In the light of these findings a pilot-plant was built to investigate 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 wastewater treatment systems, it was decided to base the lime treatment technology on that used successfully throughout the world (in both industrialised and non-industrialised countries) to treat potable water; namely 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 problems were encountered. Effluent quality data has been reported previously. This paper summarises the data and describes the practical problems associated with the operation and maintenance of the lime treatment plant, and the upgrading techniques used to overcome these difficulties. The prototype plant consisted of a rapid-mix basin in which raw municipal wastewater was combined with lime slurry at a ratio that achieved a pH of greater than 11.0 after mixing; a sedimentation tank, for solid-liquid separation; and a recarbonation basin for tertiary treatment. Constant monitoring of the lime-wastewater mix pH, using a pH controller to regulate the flow of lime slurry, proved difficult. Frequent 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 within the delivery pipework. All pipework carrying lime slurry was set up in duplicate so that plant operation could continue during cleaning operations. An improved pH control system, which linked lime slurry flow to the pH of the lime-wastewater mix, was also installed. The design of the sedimentation tank was altered to aid the removal of sludge and the tank inlet and outlet were upgraded in order to minimise short-circuiting. Plant operation and maintenance were simplified by the design improvements and the frequency of mechanical breakdown was reduced. The pH control system was difficult to operate effectively because the pH probe became rapidly fouled by lime solids. However, gel-filled, low sodium-error pH electrodes calibrated in pH 11.0 buffer proved significantly more robust in the aggressive high-lime environment than did conventional combined glass electrodes. Options for further improving the design of a lime treatment plant are considered.