Ozone counters Cryptosporidium in Milwaukee
The US city of Milwaukee has used ozonation to tackle a serious Crypto problem and ended up with safe, economical drinking water for its citizens. Robert Hulsey, process engineer, Black & Veatch, Dan McCarthy, senior vice president and project manager, Black & Veatch, and Carrie Lewis, superintendent, Milwaukee Water Works, report.
Along with the addition of ozonation, modifications were also made to improve performance at both plants under separate contract. These improvements included relocation of the intake pipe to deeper water to avoid water quality impacts from surface water run-off; rehabilitation of filters; and modifications to two reservoirs, a chemical feed system, yard piping, raw water pumping, two clearwells, and flocculation basins.
Prior to ozonation, Milwaukee had used free chlorine from the plant inlet through filtration with conversion to chloramines at the inlet to the clearwell. With ozonation, the two facilities will be converted to biological filtration. Modifications to the clearwells allowed for chlorine to be added immediately after biological filtration to reduce heterotrophic plate counts followed by ammonia addition to convert to chloramines for the distribution system.
To get the ozonation project on line as quickly as possible, Milwaukee took a chance on a project approach not previously used for municipal WTWs in the state of Wisconsin - design-build.
The jv team of Black & Veatch and Alberici Construction was not only up against the clock, but also had to make sure that both plants remained operational during installation of the new facilities. Retrofitting ozone facilities into existing treatment processes at both plants required careful engineering design of large diameter piping modifications and connections to existing structures in a way that avoided outages.
A phased construction approach was combined with temporary operational modifications to keep the Linnwood plant on-line while major clearwell, reservoir, piping, and chemical feed modifications were made. Milwaukee's decision and the team's technology-led design-build expertise resulted in a successful outcome. The project was operational in 17 months and came in $11M under budget.
- Ozonation has a significant impact on DBP concentrations. While haloacetic acid (HAA) and trihalomethane (THM) levels were already low (less than 20µg/l), ozone has provided significant reductions in these values (between 30 and 50%). Total organohalides measured at the Howard Avenue plant and surrounding distribution system were reduced by over 50%. Prior to ozonation, the concentration of organohalides was typically in the 100µg/l range. Following ozonation and biological filtration, the organohalide concentration was reduced to less than 40µg/l.
- Bromate formation following ozonation was less than 5µg/l.
- Milwaukee now achieves disinfection and inactivation of Cryptosporidium at minimal operating cost. On average, ozone addition costs less than $1.60/Ml of water treated. This is well below the International Ozone Association's expected cost of $2.60 to $53/Ml treated.
For the general public, the final test is taste, odour and colour control of the finished water. Fewer complaints received than in previous years - even in the summer, when algal growth in Lake Michigan resulted in complaints to other water utilities in the area - suggest that Milwaukee's addition of ozone improved overall water quality.
The American Academy of Environmental Engineers awarded Black & Veatch a 1999 Excellence in Environmental Engineering Honour Award for its design of Milwaukee's new ozonation facilities, while the Design-Build Institute of America (DBIA) recognised Milwaukee's accomplishments through a special Water Systems Quality Project Award at its annual conference in December 1999.