Twelve years after the first full scale municipal application in Europe of the membrane bioreactor (MBR) technology, the process is now accepted as a technology of choice for wastewater treatment, and the market is showing sustained growth. However early misconceptions about the technology are persistent and false statements are commonly encountered in articles and conferences, generating unnecessary research efforts or even fuelling either fascination or scepticism with regards to the technology, which is ultimately detrimental to the perception of the process by water professionals. We try to provide some factual and rational clarifications on ten issues which are often wrongly reported about MBR technology.

This paper deals with the performance and the optimisation of the hydraulic operating conditions of the A3 Water Solutions flat sheet membrane technology in a MBR pilot-plant to achieve a satisfying fouling control and also a reduction in the required aeration. Two vertically stacked modules were tested at pilot-scale at Anjou Recherche under typical biological operating conditions (mixed liquor suspended solids concentration (MLSS) = 10 g/l; sludge retention time (SRT) = 28 days; food to microorganism ratio (F/M) = 0.12 kg COD/kg MLSS/d). The use of a double-deck and of specific backwashes for this membrane technology enabled to achieve satisfying membrane performances for a net flux of 25 L h-1m-2, 20°C at a low specific aeration demand per membrane surface (SADm = 0.2Nm3 h-1m-2) which corresponds to a specific aeration demand per permeate volume unit (SADp) of 8Nm3 air/m3 permeate, which is lower than reported for many commercial membrane systems. The mixed liquor characteristics (foaming, MLSS concentration) appeared to influence the fouling behaviour of the membranes but no correlation was found with the fouling rate. However, with the new operating conditions, the system is robust and can cope with fouling resulting from biological stress and daily peak flows for MLSS concentrations in the membrane tank up to 18 g/l.

Given the important cost reduction of the membrane bioreactor technology in the last years, this advanced treatment process has now become cost-competitive with other conventional technologies. A cost estimation analysis undertaken with few remaining unsewered and remote areas of Berlin showed that the implementation of semi-central sanitation scheme with a local membrane bioreactor plant would lead to similar costs than the connection to the central sewer, but with a superior effluent quality. For such small systems, some design issues have to be considered in order to optimise the costs and the operation regime, such as plant capacity increase, buffer capacity, process configuration and membrane flux.

Two configurations of membrane bioreactors were identified to achieve enhanced biological phosphorus and nitrogen removal, and assessed over more than two years with two parallel pilot plants of 2m³ each. Both configurations included an anaerobic zone a head of the biological reactor, and differed by the position of the anoxic zone: standard pre-denitrification, or postdenitrification without dosing of carbon source. Both configurations achieved improved phosphorus removal. The goal of 50mgP/L in the effluent could be consistently achieved with two types of municipal waste water, the second site requiring a low dose of ferric salt ferric salt < 3mgFe/L. The full potential of biological phosphorus removal could be demonstrated during phosphate spiking trials, where up to 1mg of phosphorus was biologically eliminated for 10mg BOD5 in the influent. The postdenitrification configuration enabled a very good elimination of nitrogen. Daily nitrate concentration a slow as 1mg N/L could be monitored in the effluent in some periods. The denitrification rates, greater than those expected for endogenous denitrification, could be accounted for by the use of the glycogene pool, internally stored by the denitrifying microorganisms in the anaerobic zone. Pharmaceuticals residues and steroids were regularly monitored on the two parallel MBR pilot plants during the length of the trials, and compared with the performance of the Berlin-Ruhleben WWTP. Although some compounds such as carbamazepine were persistent through all the systems, most of the compounds could be better removed by the MBR plants. The influence of temperature, sludge age and compound concentration could be shown, as well as the significance of biological mechanisms in the removal of trace organic compounds.