The increase in greenhouse gas emissions change the precipitation behaviour causing either extreme wet- or dry weather. This impacts the fresh water supply for clean water production. Evides B.V., one of the Dutch water companies, anticipates to these changes with the sewer mining concept called ‘RINEW’ (Rotterdam Innovative Nutrients and Energy Watermanagement). RINEW studies various ways to recover valuable substances in direct decentralized sewage treatment. The treatment scheme involves the pre-treatment microscreen, coagulation and ceramic Microfiltration (cMF) for the water purification step Reverse Osmosis (RO). Since the application of cMF for sewage treatment is a novel concept, the objective of present thesis is to gain insight into technical aspects of cMF (a cMF fouling indicator and effect of oxygen on RO biofouling) and potential financial feasibility of the RINEW concept in the Netherlands.
To study the potential of a fouling indicator for the cMF, irreversible fouling rates and feed water quality results are obtained over a two month period. The feed water quality is digitally monitored and characterized on COD, NTU and EC. The parameter ‘COD’ (chemical oxygen demand) is considered the most suitable as fouling indicator due to the relative large particle size of organic matter and sticky properties of biopolymers. Relating the fouling rates to the feed water quality results in a linear trend where higher irreversible fouling rates occur at higher COD concentrations. Yet, the trend result is scattered significantly since the operational flux was highly instable over time. The feed pump is originally designed for a Nanofiltration (NF) application which may explain the deviant cMF operation.
The effect of oxygen in cMF permeate is studied on the (bio)fouling development on spiral wound Reverse Osmosis (RO). The study is done in duplicate each time using two parallel Membrane Fouling Simulator (MFS) which are fed by cMF permeate. In one MFS setup the water is depleted from oxygen by adding Sodium Bisulfite (NaHSO3). The pressure drop results in the MFS fed by aerobic water indicate a significant unstable fouling rate. The fouling rates obtained from the MFS fed by oxygen depleted water compare to similar studies done with NF permeate and tap water. This suggest a significant stable RO biofouling can be obtained if Sodium Bisulfite treatment is applied to RO feed water.
The potential financial feasibility is studied via a concept study. Concept 1 is the RINEW concept providing high quality water (e.g. demi water) to an industry. Since it is a sewer mining concept the water transport costs are neglected. In concept 2 an equal high quality water flow is produced from secondary effluent by a MF/UF+RO combination at a central Sewage Treatment Plant (SWTP). The water transport from the central SWTP to the industry is taken into account in concept 2. The difference in specific costs [EURm-3] between the two concepts is estimated which is recalculated into a breakeven the high quality water transport distance of approximately 20 km. In other words, if an industry demands high quality water and sewage is the only water source, concept 2 is more financial feasible within a range of 20 km from a SWTP. In relation to the Netherlands the SWTP density is discussed as too high for the RINEW concept to be financial feasible. Similar realized treatment plants in the Netherlands show higher importance to aspects like water source, current expertise and existing facilities in the decision-making of treatment plant.