Verfahrenskombination von Nanofiltration und Adsorption an Pulverkohle zur kontinuierlichen Abwasserreinigung

摘要

Highly contaminated waste waters (e.g. wastewater from textile dying processes or dumpsite leachate) have to be treated because of their high content of poorly biodegradable organics. A treatment process has to provide both a low cost sink for the organic contaminants and a near-complete removal of critical components. Single step processes can achive only one of these goals. Powdered adsorbents have to be used in high (i.e. expensiv) quantities for complete COD-removal (chemical oxigen demand) resulting in little loadings of the adsorbent (mass of eliminated contaminant per mass of adsorbent). Basically, nanofiltration (NF) presents a promising alternative for rejecting COD – but a highly concentrated brine is inevitable. For reducing the concentrates high recovery rates / concentration factors and, consequently, high COD concentrations are mandatory. But high concentrations lead to excessive osmotic pressures and thus reduced driving forces in the membrane process – low permeate fluxes need to be compensated with high membrane areas. Furthermore, high concentrations result (constant membrane rejection assumed) in higher permeate COD concentrations. As a result, the reduction of the concentrates is limited because of economical and ecological reasons. By performing adsorption on powdered adsorbents – e.g. lignite coke dust (LCD) or powdered activated carbon (PAC) – in an NF-module a new hybrid process was developed, which avoids the weaknesses of the single step processes while fully using their strengths: NF as a COD and solids barrier and the adsorbent as a cost effective sink for the organic contaminants. It could be proved experimentally that the proposed process enables reduced discharge concentrations, higher recovery rates / concentration factors and improved permeate fluxes compared to conventional nanofiltration applications. Based on experimental investigations in laboratory- and bench scale a plant for the treatment of 10 mä/h dumpsite lechate was designed. This capacity is identical with the size of the installed plant at the dumpsite Alsdorf-Warden from which the leachate for the experiments was taken. The specific treatment costs of the NF with upstream / combined adsorption are with 44 DM/mä approximately 50% of the treatment costs in Alsdorf-Warden if lignite coke dust is employed. If powdered activated carbon is added to the leachate upstream of the NF an AOX-concentration in the permeate can be reached that is 50% of the discharge limits at nearly identical treatment costs (46 DM/mä). But if the discharge limit for AOX – that presents the critical parameter for plant design – can already be reached with a traditional NF (without upstream / combined adsorption) this solution is recommended because of the lowest specific treatment costs of 43 DM/m3.