The City of Dallas has an aging sanitary sewer network of 3600 miles in total, considerable SSOs after each rain event, and many pipes that can be neither lined nor burst for upsizing. Add to this a population that will continue to grow rapidly from the current one and a half million people, and many wastewater engineers will recognize the situation and its challenges.
"We faced up to a key decision a few years back," explains Jun Battad PE, Project Manager within the City of Dallas Pipeline Program. "Our work program could either be purely reactive to problems as they occurred, which was pretty much how it had been before, or we could improve our way of working and thereby get more control of our pipeline program, becoming proactive in capacity management. We?d carried out some studies where the consultants used modeling, and we saw that modeling was key to gaining a better understanding of the network and through that better control of network development. I was tasked with answering two questions: should we develop in-house modeling expertise to work with models or should we outsource the modeling and utilize it only on an 'as needed' basis; and what modeling platform was best for us."
The first answer was that having an in-house team of staff that could undertake modeling to complement the work outsourced to consultants made clear financial sense. So, given this, which modeling package would be best? "Our selection criteria for the modeling package were first, a powerful dynamic model, with the ability to handle RTC, then excellent usability, and also - absolutely essential of course - very good support," says Jun. "The first criterion of true dynamic modeling shortened the list to just two, and the other criteria pointed to Wallingford Software."
In 2001 Jun became the first engineer in the US to specify Wallingford's newly-released InfoWorks modeling package for an Inflow/Infiltration study of the Hickory Creek and East Pleasant Grove basins. The specification was to model 100% of the pipe network rather than a skeletonized version covering only the larger pipes as would be normal if using a lesser modeling package. The study examined 745,000 linear feet of pipes, and included the novel technique of using radar rainfall figures for model calibration. The study data and results, proposing that more than 10% of the network needed rehabilitation or replacement at a cost estimated at $9m, have been handed over to the Water Utilities department. The value of having a small in-house modeling team is now underlined, with the ability to examine a number of new options to bring the cost down. For example, InfoWorks enables modeling of Real Time Control options, which can often improve network operations without extensive capacity increases, thereby saving money. These and other design options can be evaluated to ensure that the best value for money is obtained in any network development.
Managing this balance between a sound network with minimum SSOs on one side and ever tighter budget constraints on the other is an issue that is here to stay, underlined by the CMOM program of the EPA (the US Environmental Protection Agency). CMOM, Capacity, Management, Operations, and Maintenance, is a regulatory proposal that will require that sanitary sewer collection systems meet certain standards. On the C side, the Capacity of networks, the requirement will include that networks are capable of handling a 5 year rain event without any SSOs.
Jun sees modeling with InfoWorks as a key tool in approaching the CMOM requirements. "CMOM will impact all sanitary sewer utilities, including us. We?ve always had specific operational requirements from the TCEQ (Texas Commission on Environmental Quality), but CMOM looks as though it will be more stringently applied. The old ideas of simply using best judgement to plan sewer network improvements just won't be good enough to meet CMOM requirements. More formal methods of budgetary control and Best Management Practice will be required, and modeling is one of these essential formal tools. I don't see how any utility can approach the requirements of CMOM without a good model of their network. I'm very pleased that we have already undertaken this modeling groundwork, putting us in a good position to start work on the CMOM requirements in preparation for their imminent arrival."