Introduction

The drinking water treatment plant Hitfeld of the Stadtwerke Aachen AG (STAWAG), Germany, processes 2.7 Mm3 reservoir water per year. After flocculation and pH adjustment the raw water passes a recirculation filter and a dual-media sand filter. Backwashing of these filters consumes about 10 % of the treated water. Since the processing capacity of the treatment plant exceeds the annual contingent of raw water from the dam, standstills of the plant may occur in dry years. In order to achieve a better plant utilisation and to avoid the costs of the wastewater, recycling of the filter backwash water is highly desirable.

Before recycling, the filter backwash water is stored in a basin for sedimentation. Until 1997 the clear water from the settling tank was then directly returned to the raw water. In compliance with a recommendation of the Drinking Water Commission of the Umweltbundesamt (Ministry for the Environment) the reuse of this water as raw water was stopped and the clear water was drained. Today the reuse of filter backwash water requires the separation of solids and moreover the complete removal of microorganisms and parasites.

The required secure retention of microbiological parameters cannot cost effectively be guaranteed by means of conventional technologies. The discussion of reusing the backwash water today often results in considering the use of membrane technology1,2. Therefore on behalf of STAWAG a pilot-scale ultrafiltration plant was operated by the Institut fur Verfahrenstechnik (IVT) in co-operation with Rochem UF-Systeme GmbH, Hamburg (Rochem). The promising results of the pilot study led STAWAG to decide to treat the backwash water with a full-scale ultrafiltration plant. Here, results from the pilot study as well as first experiences with the full-scale plant will be presented.

2 PILOT-SCALE EVALUATION 2.1 Research Objectives

From the middle of June until the middle of August 1998, a pilot plant was set up in the drinking water treatment plant in Hitfeld in order to investigate the operational possibilities and the efficiency of ultrafiltration for the treatment of filter backwash water. The pilot plant was bypassed to the clear water disposal. Two full-scale modules treated corresponding to the incorporation of the future full-scale plant (Figure 1), the clear water.

Flocculation filtration I Multilayer filtration |

Flocculation filtration I Multilayer filtration |

Figure 1 Incorporation of the membrane plant into the drinking water treatment process

Only working under real process conditions, including water quality and daily standstills caused by the necessary time for filter backwash and settling, give reliable results to the questions regarding the possibility of the treatment of the filter backwash water with the use of membrane technology. The aim of the pilot study was to provide results about the efficiency of a membrane plant, the optimisation of the plant operation and the establishment of design data for a full-scale plant. Therefore, the following questions had to be answered:

Which membrane material is most suitable for this application and can guarantee the production of the desired quality of filtrate in the long term? Which flux can be obtained on a long-term basis?

What are the optimum conditions for the operation of the plant and what recovery rate can be obtained?

How can the module flushing be fitted on to this application and how often are chemicals required for cleaning or disinfection?

How does the daily standstill of the plant influence its operation?

2.2 The Pilot Plant

Rochem developed the FM module system applied for the piloting in co-operation with the IVT. The principal elements of the FM module are membrane cushions, each consisting of two rectangular membranes; two internal permeate spacers, and an incorporated carrier plate, which are welded by an ultrasonic method on the outer edges (Figure 2). A membrane element consists of these membrane cushions, which are stacked one on top of the other supported by two pins with a gap in between. These cushions are enclosed in two halves of a shell. In addition to fixing the membrane stacks, the guide pins, in combination with corresponding bore holes in the half-shell elements, carry off the permeate3.

Membrane cushion |

Figure 2 Rochem FM Module System

In the pilot plant eight of these membrane cushions elements are connected in a series and installed in a PMMA pressure tube. The pilot plant consists of two modules each fitted with about 7m2 membrane area. Both modules can be operated separately and are installed vertically to allow for effective air flushing. In order to optimise the plant operation the pilot plant can be run in manual or automatic mode, where values for the main process parameters can be chosen separately for each module.

Since solid matter concentration in filter backwash water - especially after sedimentation - is quite low, dead-end mode was chosen for this application. This means that during filtration the water is flowed perpendicularly through the membrane. The module is run as a two-end-module. The retained solid matter forms a fouling layer on the membrane surface and, gradually, builds up a flow resistance, which adds to the flow resistance of the membrane. To avoid a decreasing permeate flux, the pressure difference has to be increased during the filtration cycle.

When the filtration cycle is complete the fouling layer has to be removed, i. e. the module has to be cleaned. The long-term stability of a dead-end filtration process relies on the efficiency of the cleaning procedure. The configuration of flat channels in the FM module allows a very efficient cleaning method consisting of a combination of feed sided air bubble flushing and back washing of permeate through the membrane. The air injected into the raw water at the base of the vertically installed modules induces high shear stresses and removes the fouling layer from the membrane surface. The subsequent short period of cross-flow flushing carries the detached particles out of the module.

2.3 Results of the Pilot Study

The pilot study was begun in early June 1998. At the beginning, both modules were

Membrane cushion |

Membrane Module Cutting Tools

Figure 2 Rochem FM Module System equipped with polyaramid (PA) membranes. At the end of July polyacrylonitrile (PAN) membranes replaced the membranes in one of the modules. Both membrane materials had a cut-off of 50 kD.

The investment costs of dead-end membrane plants make up for the biggest part of the total specific treatment costs. Thus, increasing the plant capacity by enhancing the permeate flux means a substantial reduction for the total treatment costs. Therefore, the most important aim of the pilot study was to find out the highest possible flux at stable conditions with optimised process parameters. The parameters, which needed to be optimised, were the filtration time and all parameters related to the module flushing.

The start up of the pilot plant occurred at a relatively low flux performance. The flux could be raised by gradually adjusting the module flushing parameters to this application (Figure 3). Reasons for drops of the transmembrane pressure-difference (TMP) at constant fluxes were usually due to extra module flushes with air and water which were carried out in order to optimise the module flushing. As can be seen in Figure 3 it was possible to increase the performance of the PA membrane to over 150 l/m2h.

After membrane replacement the flux had to be reduced due to a rapid increase of TMP. Obviously the PAN membrane material can only be run at a somewhat lower level of flux. Applying a relatively high TMP a flux of about 115 l/m2h was possible at stable conditions with the PAN membrane.

During dead-end filtration the amount of produced filtrate equals the content of feed. Only the filtrate used for backwashing and the feed used for the cross-flow flush reduce the recovery rate. After a short start-up phase the plant reached a recovery rate of about 96 %. Recovery rate as well as energy consumption and the net permeate flux depends on the filtration time. At the end of the pilot study the filtration time for the PA membrane was 120 min and for the PAN membrane 100 min, altogether resulting in a high net flux and recovery rate at low energy consumption (Table 1).

It must be mentioned at this point, that during the three months of piloting, no chemicals were used for backwash enhancement or for chemical cleaning. The high fluxes were achieved only by adapting and optimising the module flushing procedure for this application.

The membrane material chosen to equip the full-scale plant was the PAN membrane. This was due to doubts as to whether the membrane cushions made from PA could guarantee long term integrity. Especially the welded edges of the PA membrane cushions were susceptible to damages. It was observed during the pilot study that the performance of the PA membranes regarding permeate quality was inferior to the PAN membranes. The quality of the permeate was determined by continuous particle counting and by plate counting. Although both membranes always showed drinking water quality and low particle counts there was a difference between the two materials. The retention of CFU for PAN membranes was about one log-stage better, and particle counts (>1 nm) for PAN were always below 0.5 particles per ml while the counts for the PA membrane were between 0.5 and 1.5 particles per ml.

Figure 3 Membrane performance in the pilot study Table 1 Operating data of the pilot plant o 11 "111 "1 r "111 "111 i11 i1 "111 "1 r "11 " " i1 "11 " i11 " " i1 "111 " ' i " " r "11 °

Figure 3 Membrane performance in the pilot study Table 1 Operating data of the pilot plant

Operating parameters

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