Improvements flow through
The environmental benefits of reverse osmosis can often be offset by the impact of the process. Mike Jefferies discusses the developments Avista has made to lessen its negative effects.
The environmental benefits of reusing water and reducing contamination by using membranes can often be offset by the polluting effects of the technology.
In seawater RO demineralisation, for example, these include:
- Energy use (typically 3kWh/m3)
- Brine generation (derived from the rejected dissolved solids)
- Discharge of membrane treatment and cleaning chemical residues
Typical in RO, several dosing agents are used. Chlorination and de-chlorination might be used for disinfection; coagulants to improve colloid removal; several other agents, including antiscalant, biocide and cleaner, are dosed to maintain the membrane performance.
These are essential to maximise membrane lifetime, minimise plant down time and keep power requirements to a minimum. Without anti-scalant, for example, membranes rapidly deteriorate.
The main environmental contaminants commonly used in dosing agents are phosphate, linked closely to eutrophication of freshwater and the coastal marine environment, EDTA chelating agents, and other chemicals with poor biodegradability.
The combined aquatic toxicity of the formulations compound the problem. These concerns have been identified by Ospar, an international marine pollution commission.
Over the past decade, operators and plant designers have reduced harmful emissions through operational changes. These include using intermittent rather than continuous dosing, and synergistic chemical blends that use amounts of several formulations, including biocides. But the improvements achievable by dose minimisation are finite if RO treatment is to remain effective.
Avista Technologies has created formulations lower in key contaminants (EDTA and phosphorus), and with better biodegradation. It says its Vitec SR antiscalant improves biodegradability and reduces phosphorus inputs from this source by up to a third, while its RoClean P111C membrane cleaner has eliminated EDTA and other components with low degradability by incorporating non-toxic, biodegradable alternatives.
The approach taken by Avista's chemists focused on four key criteria:
- The new formulation needed to be as effective as its predecessor and economically viable
- Formulations had to be compatible with all RO membranes both alone and in combination with other dosing agents
- The formulation had to be easy to handle, and have a good shelf-life
- Ospar guideline compliant: better than 20% biodegradability; aquatic toxicity >10mg/l; an octanol-water partition coefficient lower than three
Development of an environmentally friendly membrane cleaner focused on how best to remove harmful EDTA. In recent years, a range of chelating agents have been developed. Candidates were first screened for compatibility with membrane materials.
Tests compared cleaning effectiveness, and confirmed compatibility with membrane materials. The most promising were then tested on full-sized membranes to assess cleaning effectiveness.
The next stage tested the formulations on fouled membranes. The best performing formulation from 12 repeat trials was evaluated against Avista's existing P111 cleaner.
The new formulation, called RoClean P111C, is more effective that its predecessor, EDTA-free, and has significantly better biodegradation, now meeting all Ospar guidelines. P111C has now proven successful for over a year in operational use.
Development of an improved antiscalant required a complete reformulation.
Avista's chemists identified two chemicals with the desired characteristics and carried these forward to the pilot testing stage.
A six-month trial using a pilot plant under realistic conditions was used. Running at up to 350l/h for 1,000 hours, the formulations were tested for long-term effectiveness against common scale inhibitors. They were also assessed for effectiveness against biofouling, along with compatibility testing against a range of membranes. The stability of these formulations were also established.
Data on permeate flow and pressure differentials enabled the quantification of membrane fouling. Both formulations performed well, maintaining a near optimal normalised flow rate for periods in excess of 40 days.
One formulation, Vitec SR, was preferred because of its overall lower dosing requirement. This was next trialled on a seawater membrane site. After meeting Ospar criteria, and confirming its compatibility with the membranes used at the plant, a month-long trial confirmed Vitec SR was as effective as the previously used antiscalant.
No deterioration in normalised permeate flow-rate was observed, while an improved environmental performance, a greatly improved biodegradation (>25%) and lower phosphorus content (4%) was achieved. Furthermore, laboratory and field data show the formulation is effective against scaling at brine concentrations 20% higher than typically experienced.
Seawater RO plants produce drinking water from a non-depleting source. Operational improvements have seen a lowering in the quantities of the polluting chemicals the process requires. Now, further environmental improvements have been made by Avista through the development of their membrane cleaner, RoClean P111C, which is free of harmful EDTA and biodegrades to a greater extent than prior formulations.