Water Treatment Plant (WTP) Control Strategies for T&O

T&O compounds can be produced within WTPs

Algae growing on the walls, bottoms, and weirs of WTP basins can be a source of T&O production.  Pulse addition of chlorine or copper can be effective in controlling this in-plant T&O production.  Pulses should be every 2 weeks when water temperatures are greater than 20 oC.  Addition of powder activated carbon (>3 mg/L) to the presedimentation basins produces a high degree of light-shading and prevents signficant growth of algae in basins with PAC.  Algae growth within WTPs pose both an operation problem and potential to form T&Os.  Algae “mats” can clog wiers and algae cells of all types reduce filter run times.   No T&O production within C.O.P. WTPs have been observed, but MIB production within the WTP was observed by the City of Tempe.  Therefore, it is important to control the growth of algae within WTPs.  The following techniques are recommended with certain qualifying statements:

Prechlorination treatment during periods of warm water (>20 oC) could be conducted once every week for algae control.  Prechlorination doses are selected to give a residual prior to additional chlorine addition near filters.  Direct filtration and conventional WTPs in the metro-phoenix region only remove 5%-15% of the TOC.  TOC can react with chlorine to form DBPs (THMs and HAAs).  Therefore, delaying the point of chlorine to after TOC removal (after sedimentation basins) would be advantageous.  Prechlorination for 24 hours increases DBP levels leaving the WTP for that period.  Prechlorination should not take place concurrent to PAC addition; chlorine reacts with the PAC and reduces its effectiveness to adsorb MIB or Geosmin.  Prechlorination has not been observed to break algae cells resulting in release of MIB or Geosmin.

Liquid copper products (e.g., copper sulfate, Earthtec) can be feed (0.3 to 0.8 mgCu/L) at the head of presedimentation basins to reduce algae growth.  Copper is toxic to algae at low levels.  Copper addition for 12 to 24 hours once every nine to 14 days should control algae growth.  Excessive copper addition can lead to growth of copper-resistant algae, and accumulation of copper in WTP and wastewater sludges.  Recommended copper doses are below action levels for the Lead and Copper Rule.  Copper addition has not been observed to break algae cells resulting in release of MIB or Geosmin.
T&O compounds can be removed within WTPs

Powder activated carbon addition

Visit the PAC specific webpage: click here for PAC details

Biological filtration

Granular activated carbon (GAC) capped filters operated in an adsorption or biologically active mode will remove MIB and Geosmin.  Existing anthracite filter caps would be replaced by GAC caps.  The GAC caps should be 30- to 50- inches in depth.  The point of chlorination should be after filtration, which could affect CT disinfection credits.  Depending upon operating conditions 20% to > 90% MIB and Geosmin removal can be achieved.  PAC addition may not be required when operating in adsorption modes only, while it would be required under biologically operated (exhausted adsorption capacity).  GAC caps operated under adsorption mode, and to a lesser extent under biological mode, would provide TOC removal and removal of synthetic compounds (e.g., estrogenic compounds and pharmaceuticals).  WTPs with short presedimentation contact times for PAC and/or high influent T&O concentration would benefit most by GAC-filter caps.  GAC filter caps add another layer of treatment in a multiple-barrier approach to T&O control.

Ozonation and AOPs

Ozone and UV irradiation can be effective at removing MIB and Geosmin.  UV irradiation dosages are required for MIB or Geosmin oxidation are approximately 100 times greater than dosages used for microbial inactivation.  Therefore, UV irradiation will probably is not cost-effective.  Ozone dosages capable of Giardia and Cryptosporidium inactivation (2 to 4 mg/L) are capable of 80% to > 95% oxidation of MIB and Geosmin in CAP or SRP water.  Ozone dosages of 2 to 4 mg/L will form bromate, and depending upon the initial bromide concentration in the raw water (80 – 150 mg/L), bromate concentrations approaching the MCL of 10 mg/L would be formed.  Therefore, acid and/or ammonia addition prior to ozonation would be required.  However, if ozone was used primarily for T&O control, lower ozone dosages (e.g., 0.75 mg/L) could achieve modest MIB removal (e.g., 60%-80%).

Chlorine Dioxide