Hamilton Kent Blog

Easy determination of I&I in sanitary system

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We’ve got a really (REALLY) easy way to determine how much I&I is in your sanitary system!


I&I accounts for almost half of what you treat!


Municipal and federal agencies in Canada and the US have long recognized the problems associated with I&I (inflow and infiltration) in separate sewage collection systems.   Very little precise information is available on the extent of I&I as a national problem. So how can managers and regulators ever hope to formulate strategies to improve their systems and benchmark their performance?


George Kurz, P.E., DEE, an independent engineer in Madison, Tennessee is currently studying this problem in a pretty unique way with the intention of determining the quantity, scope, and characteristics of I&I for the entire state of Tennessee.  George utilizes readily available data routinely recorded by plant operators in MOR’s (Monthly Operations Reports) and specifically evaluates daily influent flow, influent organic load, and rainfall for one year.


Here’s George’s basic explanation of how he works his magic:


“Recent guidance documents from EPA and The Federation of Canadian Municipalities describe an approach for estimating I&I which relies on hourly flow monitoring. However, most treatment plants in Tennessee record flows from chart recorders or totalizers on a daily basis – this has been done for years. This study demonstrates a simple approach that uses data already routinely collected daily by plant operators. It includes the traditional hydrologic approach and also estimates the effect of I/I dilution.”


George goes on to explain that;


“Average BOD concentrations in the influent conveyed to plants in Tennessee varied from 32 to 893 mg/l. The instances of high concentrations of BOD were usually associated with known industrial discharges. Other investigators found that undiluted residential BOD concentrations averaged around 350 mg/l. EPA used this figure as an average concentration for undiluted residential BOD in its guidance for design of pressure systems and small alternative treatment systems. When the Tennessee influent BOD levels were analyzed to estimate the amount of I&I dilution, about a third of the total I&I was attributable to dry weather infiltration, which was not detected by flow analysis alone. In a few cases, infiltration may be overestimated due to significant, diluted industrial flows or flows from combined sewer systems.”


In plain language, he is calculating how much clean water is leaking into the sanitary system based on how diluted the sewage becomes when compared to dry weather concentrations – in conditions when presumably there is zero inflow occurring. 


Simply put and most importantly to cash strapped municipalities struggling with this problem, this approach does not require installation of additional equipment or costly engineering studies!


George is about halfway through Tennessee’s 227 municipalities – and early results show that 45 per cent of the sewage those systems are treating is clear water on an annual basis.  I&I represented more than half the annual flow in two-thirds of those systems. Projecting this leakage rate across all systems in Tennessee results in an annual I&I estimate of 112,500 million gallons (425 million m3) for the state.


Based on the current results at the halfway point of the study, I&I represents an annual cost greater than $200 million in Tennessee. This cost assumes a uniform O&M cost of $1.80 per thousand gallons (or $0.47per m3, which is very conservative when compared to the EPA’s 2014 recommended O&M rate of $2 to $5 per thousand gallons. It’s important to also note that this cost does not include the “capital costs” - specifically for the sewer lines, tanks and wastewater treatment plants needed to convey and treat wastewater.


In addition to annual I&I, George is also estimating the effects of RDII (rainfall derived I&I) and long-term dry weather infiltration for each system. According to George, the results show that 89 per cent of the plants may exceed the nominal capacity listed in their permit when the RDII flow increases for a 2-year storm (about 3.4 inches or 8.6 cm in 24 hours for middle Tennessee). While that figure is greater than expected, it is not necessarily cause for alarm since most plants have additional capacity to absorb peak flows – however, the old maxim “just because you can, doesn’t mean you should” applies.


The good news? This situation can be reversed and the savings would offset the costs for rehabilitation.


George indicates that this approach of using treatment plant flow, rainfall and influent characteristics from MORs is not intended to replace comprehensive monitoring and engineering studies to evaluate I&I and RDII. Instead, the intention is to very effectively analyze all the collection systems in the state with a uniform procedure using existing data and a little math to provide meaningful and useful information to program managers and decision makers.


George has created Excel analytical spreadsheets which do the math, so would be useful to individual operators of public systems as an “I&I calculator.” Small communities are often reluctant to spend any money for additional equipment or engineering studies. This tool provides an opportunity for operators to help themselves to determine the presence and extent of I&I in their systems.


We encourage anyone interested in learning more to contact George directly.


If you want every little bit of detail, click here!


Find out how Hamilton Kent’s Lifespan System – a truly watertight manhole frame and cover - can play such an incredible role in reducing your I&I and use the savings to pay for the project capital costs – usually in three years:


Watch Dan Leger’s H2OMG webinar