Membrane methods in tailoring simpler, more efficient, and cost effective wastewater treatment alternatives

The continuing development of higher performance selective membranes of reverse osmosis, RO, and nanofiltration, NF, enables the elaboration of separation processes and wastewater treatment methods based on simple or hybrid membrane processes. The present work reports on the continuation of a long term investigation [1] of the recovery of certain grades of industrial wastewaters by NF and/or RO in comparison with the classical procedures of treatment, or in combination with them, so as to determine the optimum process efficiency and cost. Treatment of RO reject stream high in hardness content, with NF in comparison with the conventional hot lime softening revealed similar efficiency in hardness rejection by NF in addition to (1) parallel water desalination, (2) absence of chemical dosing stoichiometric to hardness removal (3) absence of sludge formation (4) absence of steps of slow kinetics, and (5) absence of need to subsequent water filtration. Recycling of NF filtrate to the initial RO feed could raise the total process recovery to 95% with only minor additional cost. On the other hand, separation of trace heavy metal cations, HMC, Cd2+, Ag+, and Hg2+ from mixture salt solutions, was investigated by NF in comparison with their conventional removal by precipitation as hydroxides or sulphides, or by means of chelating ion exchange resins, under the same conditions. Comparative evaluation of results revealed the high efficiency of NF which attained 98% separation at recovery above 90%, and confirmed the absence of interference with rejection of the similar valency cations despite their much higher concentration. In addition to the above mentioned advantages, NF enabled the optimum recycling of the permeate as process water, and the safe disposal of the minor volume reject, after precipitation of the HMC to the sewer stream.

Innovations in selective membrane technology and the continuous development of more performant membranes of RO, NF, and UF enables the progressive replacement of conventional wastewater treatment processes and the introduction of more efficient cost effective, easier, and environmental friendly simple or hybrid membrane treatment processes. (1) Rejection of hardness components by NF shows the following advantages over the conventional hot lime softening method: (a) Parallel general desalination. (b) Absence of continuous chemical dosing specific to softening and sludge formation. (c) No sophisticated installations or equipment are required. (d) No slow steps (coagulation, settling) are included and no subsequent filtration is required.