Poor management of feedwater and membranes can significantly raise operating costs. Monthly chemical cleanings, when every six months or once a year would do. Annual membrane replacements, when every five years would do. This has a direct impact on costs and productivity.
Preserving the Membranes: First Step
The first step in preserving membrane performance is removing chlorine or any other oxidizing biocide. TFC (Thin Film Composite) membranes are sensitive to these compounds and degrade quickly in their presence. Removal can be done with sodium metabisulfite. Microbiological control should be handled with non-oxidizing biocides, such as isothiazolinone or DBNPA.
SDI — Silt Density Index
One way to quantify the impact on membranes from suspended solids loading is the SDI (Silt Density Index), standardized under ASTM D4189. This parameter quantifies the membranes' tendency to foul.
SDI is controlled by the water treatment plant. Poorly managed operations will have a significant impact on membrane productivity, maintenance, and replacement. SDI should be monitored at least daily — not only during occasional audits.
Estimated cost of an 8" x 40" spiral-wound membrane element: BRL 1,200 to BRL 2,500 (USD 230–480). Industrial systems with 50 to 100 elements represent BRL 60,000 to BRL 250,000 in membranes. Early replacement due to preventable fouling has no technical or economic justification [ASTM D4189; DuPont FilmTec Technical Manual, Form No. 609-00071].
Membrane Fouling
The main fouling agents are CaCO₃, BaSO₄, SrSO₄, silica, among others. As the water flows through the cartridges and is permeated, the remaining water undergoes a gradual increase in dissolved solids, which can precipitate once the solubility limit is reached.
This phenomenon can be controlled by limiting the concentration cycle and by using antiscalants. One of the mechanisms involved is the threshold effect: antiscalant molecules adsorb onto crystallization nuclei during the early stages of precipitation, suppressing it even when the solution is supersaturated.
Energy Consumption
The cost driven by the progressive rise in differential pressure from fouling increases the specific energy consumption of the high-pressure pump. These figures should be tracked through specific consumption based on normalized flow (kWh/m³), following the methodology described in the FilmTec Technical Manual (DuPont, Form No. 609-00071-1020).
References
- ASTM D4189-07(2014). Standard Test Method for Silt Density Index (SDI) of Water.
- DuPont Water Solutions / FilmTec. FilmTec Membranes Technical Manual.
- Hydranautics / Nitto. Technical Service Bulletin TSB107.12.
- AWWA Manual M46 (2007). Reverse Osmosis and Nanofiltration, 2nd ed.
- Fritzmann, C. et al. (2007). State-of-the-art of reverse osmosis desalination. Desalination, 216(1-3).