AiRO: Direct Reverse Osmosis On Surface Water

Surface water is microbiological unsafe, contains too much salts and is polluted with anthropogenic substances like pesticides, medicines, endocrine disruptors and industrial chemicals. The production of safe drinking water out of this water is only possible by applying a robust treatment. A process that can remove virtually all unwanted substances from the water in one step is reverse osmosis (RO). The main bottleneck in the wider application of RO with spiral wound membranes is the membrane fouling. This occurs on the feed spacer of the spiral wound membranes and at the membrane surface. This fouling occurs with particles, salts and/or microorganisms and results in a raise in pressure drop and reduction in permeate quality. An innovative idea to deal with above mentioned drawbacks is a membrane process in which spiral wounded NF/RO membrane elements are cleaned hydraulic (mechanically). The membrane is placed vertical and the membrane fouling is controlled by means of a periodic air/water flush, AiRO called (combination of the words air and RO). This AiRO process line needs hardly any pretreatment, is free of antiscalant dosage, uses less chemicals than traditional NF/RO and the concentrate is easy to dispose to surface water. The semi practical AiRO research described in this thesis is carried out with an automated pilot plant that was equipped with 2 8” RO membranes. The pilot was placed at the banks of the Hollandse IJssel river in Montfoort. The pre treatment was a drum filter. The pilot was started up in December 2010 and operated non stop for one and a half year. The research has shown that AiRO is a reliable robust barrier in a surface water purification process line. There is no effect on the operation of the process if the turbidity of the feed water varies between 4 and 75 FTU and if the oxygen content in the feed water varies (indication for microbiology). The process appeared to be economic interesting and withstood the practical durability test. Pre treatment of the feed water of the AiRO with a 120 µm micro strainer is sufficient. The commonly in Netherland applied 35 µm micro strainer is advised because it will give advantages for the AiRO process (less wear in pumps and energy recovery) and will promote the surge for a solution for the concentrate. Commercial available NF/RO elements are applicable in AiRO and the membrane stack is composed with pressure vessels with 2 8” elements in series. The investment costs of a complete drinking water treatment with the AiRO process are 20 to 33% lower than a treatment based on UF and RO. The application of 16” pressure vessels containing 2 16” elements in series may result in even lower investment costs. A flush was executed with the maximum allowed feed pressure (specs. Membrane manufacturer), the flush water velocity was 0,2 m/s in the membrane feed spacer, and with an air velocity of 0.24 Nm/s. Energy recovery from concentrate (Fedco) is essential to obtain energy efficiency. The flush program that resulted in a stable operation of the process consists of the steps. The flush interval is 4 hours and pressure drop NPD is the parameter for fine tuning of the interval duration. A flush should start if the NPD doesn’t rise linear anymore. A CIP has to take place, if the NPD before a flush rises above 50 kPa. The applied chemicals have to be balanced on the feed water composition. Manganese oxides by example are hardly washed out by a flush, they have to be removed during a CIP with citric or ascorbic acid. The design recovery is 35% and operation of AiRO at this low recovery results in a large concentrate stream, which is free of antiscalant and with a low content of salts. This flow is expected to be allowed to discharge to surface water. The flush water can be discharged to surface water, after sedimentation of the solids in it. The produced sludge is free of chemical additions. This will improve the useful application of the sludge. The used CIP fluid has to be discharged to the sewer after neutralisation of the pH.Sanitary EngineeringWater ManagementCivil Engineering and Geoscience