Mike Jefferies describes the development of Avista Technologies' two new environmentally friendly reverse osmosis membrane effective treatment and cleaning formulations. Vitec SR was developed as a low-phosphorus, biodegradable antiscalant, whilst RoClean P111C is a membrane cleaning formulation free of EDTA with a much improved biodegradability over its predecessor.
Reverse osmosis (RO) technologies usefully generate high quality permeate suited for potable water, water re-use and other applications, providing sustainable water use or environmental benefits from reduced discharge to the environment of many pollutants. There are, however, environmental impacts derived from using RO technology, including power use (typically 3 kWh/m3 from seawater demineralisation), brine generation (from retained dissolved solids), and the discharge of membrane cleaning and treatment chemicals. These chemicals, including biocides, antiscalants and cleaning agents, are essential in maintaining RO membrane performance, minimising RO power use, reducing plant downtime and ensuring the permeate quality remains high. Figure 1 shows what happens when these chemicals are not dosed correctly.
Figure 1
Mixed sulphate scaling rapidly causes deterioration in RO membrane permeability. Dosing agents are essential in maintaining RO membrane performance and minimising plant downtime.
Avista Technologies, a speciality chemical company solely focussed on providing products and services to assist the efficient and economical operation of RO systems, has developed and improved key RO membrane formulations to significantly reduce their environmental impact without compromising membrane efficiency. The main contaminants of concern within existing formulations, as identified by OSPAR (an international marine pollution commission) are phosphate, linked closely with freshwater and coastal eutrophication, and EDTA, a chelating agent with very low biodegradability and long environmental residence time. Operators have reduced their use of many formulations through measures such as intermittent dosing and blending, but such minimisation has a finite limit before membrane effectiveness is compromised.
Accordingly, Avista Technologies redeveloped two key formulations to reduce the contaminants of major concern. Their new Vitec SR antiscalant improves biodegradability and reduces phosphorus inputs from this source by up to a third, whilst their RoClean P111C membrane cleaner has eliminated EDTA and other components with low degradability by incorporating non-toxic highly biodegradable alternatives.
Employees at Avista offer a tremendous value added resource with their wide expertise in chemistry and formulation development for the RO market. Their approach focussed on four key criteria:
• new formulations must be as technically effective as predecessors, and be economically viable.
• formulations must be compatible with RO membranes, alone and in combination with other dosing agents.
• formulations must be easy to handle, and have a good shelf-life.
• physical and chemical characteristics of >20% biodegradability, aquatic toxicity of >10 mg/l, and log POW<3, in line with OSPAR limits.
Development of an environmentally friendly membrane cleaner focussed on the removal of harmful EDTA and components with poor biodegradability. Replacement chelants were screened for their compatibility with membrane materials; three were taken to the next development stage. After preliminary laboratory testing of formulations, the most promising were tested on full-sized fouled membranes to quantitatively assess cleaning effectiveness. The best performing new formulation (RoClean P111C) from repeat trials was evaluated against the existing cleaner (RoClean P111), verifying that the new formulation exceeded its predecessors performance. RoClean P111C achieves this improved performance, but is EDTA-free, and has significantly better bio-degradation characteristics (meeting all OSPAR criteria), whilst retaining its predecessor's beneficial features of membrane compatibility, economic viability, ease-of-use and extended shelf-life. RoClean P111C now has proven success for over a year in operational use.
Development of Vitec SR required a complete re-think on the existing antiscalant formulation. Recent advances, largely through synergistic blending of phosphonates with other antiscalants, have reduced phosphorus content of antiscalants to less than 6%. This still falls short of OSPAR criteria. So, Avista's chemists identified a number of potential new antiscalant chemistries. These were screened for membrane compatibility and effectiveness against a range of common scale inhibitors. Two chemicals were chosen for development as two synergistic formulations. A six-month trial using a pilot plant (running at up to 350 l/h for 1000 hours) under realistic conditions evaluated each formulation for its long-term effectiveness against common scale inhibitors and biofouling, formulation stability, and membrane compatibility. Both formulations performed well (Figure 2), maintaining near-optimal normalised flow rate through membranes for prolonged periods. One formulation (Vitec SR), preferred of the two because of its overall lower dosing requirement, was trialled for one month on a full-scale seawater membrane site. The trial was successful; no deterioration in normalised permeate flow-rate was observed, whilst an improved environmental performance was achieved. Vitec SR has a greatly improved bio-degradation (>25%) and lower phosphorus content (4%) than its predecessor.
Figure 2
Pilot plant trial data (typical of several repeats) showing a well maintained normalised permeate flow-rate over 40 days when the membrane is dosed with Vitec SR (“Formulation E”) anti-scalant compared to a membrane not dosed. Permeate flow and pressure differential data across the RO membrane are normalised to account for ambient pressure and temperature.
Avista's new formulations, bio-degradable antiscalant Vitec SR and EDTA-free RoClean P111C membrane cleaner, are more effective than their predecessors and significantly reduce the environmental impact of the use of these formulations.
Useful Links
Avista Technologies website - www.avistatech.co.uk
About the Author
Mike Jefferies managing director at Avista Technologies. Contact via their website.