Selection of the surface water treatment technology – a full-scale technological investigation Alina Pruss ABSTRACT A technological investigation was carried out over a period of 2 years to evaluate surface water treatment technology. The study was performed in Poland, in three stages. From November 2011 to July 2012, for the first stage, flow tests with a capacity of 0.1–1.5 m 3 /h were performed simultaneously in three types of technical installations differing by coagulation modules. The outcome of the first stage was the choice of the technology for further investigation. The second stage was performed between September 2012 and March 2013 on a full-scale water treatment plant. Three large technical installations, operated in parallel, were analysed: coagulation with sludge flotation, micro-sand ballasted coagulation with sedimentation, coagulation with sedimentation and sludge recirculation. The capacity of the installations ranged from 10 to 40 m 3 /h. The third stage was also performed in a full-scale water treatment plant and was aimed at optimising the selected technology. This article presents the results of the second stage of the full-scale investigation. The critical treatment process, for the analysed water, was the coagulation in an acidic environment (6.5 < pH < 7.0) carried out in a system with rapid mixing, a flocculation chamber, preliminary separation of coagulation products, and removal of residual suspended solids through filtration. Alina Pruss Poznan University of Technology, Institute of Environmental Engineering, Division of Water Supply and Environmental Protection, Berdychowo 4, 60-965 Poznan, Poland E-mail: alina.pruss@put.poznan.pl Key words | algal bloom, dissolved organic carbon (DOC) fractionation, full-scale technological investigation, organic matter, surface water, water treatment INTRODUCTION Surface waters are exposed to the impact of numerous pollu- tants. As a result of runoff pollution, including fertilisers used in agriculture, a continuous increase in surface water con- tamination has been observed. The resulting massive growth of aquatic plants, i.e. algal blooms, leads to secondary pollution of water. Due to an increasing concentration of water organisms and their productivity, the amount of organic substances in the water is high and the content is variable. Such substances, with various molecular weight and structure, can demonstrate different susceptibility to removal from water (Hem & Efraimsen ; Huber et al. ; Vasyukova et al. ). The main processes used to effec- tively reduce organic matter concentration during the course of surface water treatment include coagulation, sedimen- tation and filtration (Sohn et al. 7; Lawler et al. ; Wolska ). A number of factors influence the effectiveness of coagulation and filtration for removal of organic sub- stances from water. Detailed descriptions of such factors can be found in numerous publications (Pruss et al. ; Teixeira & Miguel ; Baghoth et al. ; Guminska & Klos ; Klos ). These reviews conclude that the correct way to choose the best surface water treatment technology prior to the design stage is through technological studies. Intensification of removing organic contaminants from water during the coagulation of the volume is made possible by using optimal process parameters, including the proper dosage of coagulant, the optimum pH at which the process is performed, as well as the appropriate time and mixing intensity. The required total organic carbon (TOC) percen- tage removals increase with increasing raw water TOC concentration and decrease with increasing alkalinity. Fur- thermore, natural organic substances are effectively removed at a lower pH; thus – due to the greater cost water purification alkaline pH range – the required degree of removal of TOC decreases with the increase of its alkalinity. The efficiency of the coagulation process has a signifi- cant impact on the final efficiency of the resulting water purification throughout the process system. According to many researchers, pH has a greater influence on the removal efficiency of organic matter than the type or dose 638 © IWA Publishing 2015 Water Science & Technology | 71.4 | 2015 doi: 10.2166/wst.2014.513