Since Gilbert, Ariz., entered into a public-private partnership two decades ago for the contract operations  of its Neely wastewater treatment plant, the town has enjoyed various benefits. Working with its private partner, Severn Trent Services, has enabled the Gilbert to undertake numerous capital projects and expansions, in turn implementing many cost-saving measures, including:
- 1995 – Construction of an on-demand water system;
- 1997 – Construction of a denitrification system;
- 1999 – Design and expansion of the plant to 8.5 mgd; and
- 2000 – Expansion of the plant to 11 mgd.
Implementing cost-saving measures
Upon entering into the partnership, Severn Trent used its patented MAT2000  mobile analytical trailer to evaluate the facility’s wastewater process operations. The results allowed the staff to implement several cost-saving measures, enhance the efficiency of operations, and extend the life of existing equipment.
For example, nine of 15 rotors could be taken off line during low loading periods and compliance could still be maintained, which reduced energy consumption and wear on the equipment.
MAT2000  data was also used to demonstrate that surface aeration alone was not sufficient to meet the oxygen requirements during peak load periods and on the weekends. This led to the installation of supplemental diffused air in the new ditches during an upgrade, while the old ditches were fitted with supplemental air.
When the decreased reliability of effluent filters threatened compliance, Severn Trent researched solutions for their replacement. The performance of the filters was such that new support structures had to be built in-house to keep the filters online.
Unfortunately, the manufacturer could provide no explanation for the failure and their recommended solution was unacceptable. A new supplier was located and worked with them to implement a sound solution to the problem. Severn Trent’s project manager, a registered professional engineer, designed the size and location of the new inlet openings and reviewed the supplier’s plans for the retrofit at no additional cost to the Town. The result was a more reliable, efficient process at a lower design and construction cost.
Another example of cost savings through in-house services is fabrication of new rotors for the oxidation ditches by Severn Trent’s staff.
In 2003, cracking in the hubs caused two of the original 12 rotors to fail. The cost to replace the rotors was estimated at $18,000 and normally would have been placed against Gilbert’s repair and replacement budget. Instead, two new rotors were fabricated and installed for about two-thirds of the replacement price, leaving repair and replacement funds for other critical projects.
Additional advantages to the partnership
During the last two decades, the facility has treated over 33 billion gallons of sewage. The treated effluent has been used for a variety of purposes, including recharging the town’s aquifer. The town has constructed 18 recharge ponds on over 175 acres in the urban area, and a third site measuring up to 70 acres. Given Gilbert’s shallow water table, effluent is also used to irrigate parks, golf courses and housing development common areas.
The Gilbert-Severn Trent Services partnership has received numerous accolades from the Arizona Water & Pollution Control Association, including the Wastewater Treatment Award of Merit. Severn Trent Project Manager, Dennis Plzak, was named Supervisor of the Year in 2002 and John Lydon, the project’s chief operator, won the Operator of the Year award in 2000.
As the partnership progresses into its third decade, municipalities considering a public-private partnership for water and/or wastewater services can look to Gilbert to see how such an arrangement can benefit their communities.