2003 Case History[i]
A coal-fired power plant located in New Mexico supplied power to the southwestern power grid. Water from a nearby zero-discharge lake was utilized as makeup water. This biologically active lake water was also used for cooling and was moderately warm in winter and very warm in summer. The lake had been concentrating with use and abundant nutrients for microbiological growth were present. A water analysis of the inlet water in mid-February of 2003 showed a pH of 8.4 and conductance of 1100 micro-Siemens (mS). A local consulting firm was utilized to use chlorine dioxide for bacterial control in the facility. A chlorine dioxide demand test on the lake water was measured to be ~ 1 mg/L, which was significantly higher than other, nearby lakes.
A company was contracted to build and operate the pre-treatment system. Water for the high-pressure boilers was produced from a demineralizer that polished the RO water. Two RO systems processed an average 400 gpm through FilmTec 5010 BW30-365 polyamide membranes and TriSep X201 TSA polyurea membranes in the first pass ROs and TriSep ACM2 membranes in the second pass ROs.
Early in the project, an attempt was made to control bacteria with sodium hypochlorite but was unsuccessful. Ferric sulfate was being used as a flocculant ahead of twelve multimedia vessels using anthracite and garnet/sand as filter medium.
The operating company chose to study chlorine dioxide before committing to generator installation. For purposes of demonstrating chlorine dioxide’s effectiveness in bacterial control, three areas in the facility were treated as isolated entities (Figure 1):
- Prior to multimedia filters
- Prior to the Suction Surge Tank
- Prior to the membrane E-100, 300 1st array RO (TriSep X201 TSA membranes) in first and second pass array of E-100. (Eight vessels, 1st array; four vessels, 2nd array)
Concerns over incomplete conversion of chlorite to chlorine dioxide, the possibility of traces of free chlorine existing in the produced chlorine dioxide solution, and a prior bad experience with chlorine led to several important operational decisions.
- The generator was deliberately “detuned”. That is, rather than operate on an exact balance of acid, bleach, and chlorite to maximize production of chlorine dioxide, excess chlorite was fed. The presence of excess chlorite minimized the possibility of any free chlorine existing in the generated chlorine dioxide solution.
- The solution pH was also kept at 9.0, as hypochlorite ion was thought to be less aggressive than hypochlorous acid.
- A third-party analyst was contracted to monitor chlorine dioxide and free chlorine.
- Chlorine dioxide target concentration was <0 mg/L. Chlorine dioxide was produced at a concentration of about 500 mg/L. The chlorine dioxide solution was fed into a tank and mixed with permeate water to achieve a chlorine dioxide residual of < 1.0 mg/L in the tank.
- Cleaning was done by reversing normal flow. The reason for this was that any biofilm loosened during treatment would be removed by reversing flow.
Testing before and after treatment showed that both bacteria and associated biofilm were under control.
The data appeared to support the conclusions of others, that biofilm formation on RO membranes performed like a secondary membrane which acted as a patch to cover microscopic holes in the membrane.[ii]
It may be this phenomenon that has been observed when chlorine dioxide has been applied at low doses to an already-fouled membrane. The chlorine dioxide stripped the biofilm and uncovered the pre-existing holes. Salt leakage increased, and the conclusion reached was that chlorine dioxide damaged the membrane. When chlorine dioxide feed stopped, however, the biofilm repaired itself and leakage stopped.
The net effect of this cleaning process was that cross pressures after cleaning were improved by a factor of three over the best prior historical cleaning. Photo micrographs of previous element autopsies gave the impression of “invasive tree roots breaking concrete.”
In summary three conclusions were made from this work.
1. An ounce of prevention is worth a pound of cure.
It appeared to be much easier, less time consuming, and much less costly to prevent or control bacterial biofilm buildup in an RO system than to clean up a severely fouled RO system. A good analogy for water treatment professionals is that of scale formation in boilers. It is much easier and much more cost effective to complex hardness (requires stoichiometric treatment) going to an industrial boiler than it is to try to clean the scale up once it forms (in this case treatment is not stoichiometric). Once the scale forms on boiler tubes, one can never completely remove the scale on-line.
2. Chlorine dioxide can be used for cleaning MODERATELY fouled membranes.
However, using chlorine dioxide in this way will require extensive manpower and downtime (eight-twelve days in this case) and thus should be used to remediate existing facilities during low demand and/or turnaround. (Severely fouled membranes will generally have too much damage to be cleaned effectively.)
3. Use of chlorine dioxide can reduce cost and improve effectiveness of RO operation when used correctly.
[i] Averett, W., Simpson, G., and Miller, J., “Cleaning RO Membranes with Chlorine Dioxide,” Southwest Chemistry Conference, sponsored by TXU Energy, Dallas, Texas, July 28 – August 1, 2003.
[ii] McDonough, R., Schaule, G., and Flemming, H., “The Permeability of Biofouling Layers on Membranes,” Journal of Membrane Science, 87, 199(1994).