Wastewater collection systems are under attack from peak flow I & I rates

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Normal operating conditions
Normal operating conditions.

By Robert Budway

During my 34 years working in the water and wastewater sectors I have witnessed wastewater collection systems increasingly being adversely affected by peak flow inflow & infiltration (I & I) rates/volumes. New growth and development means many communities’ infrastructure is under pressure to accommodate this additional demand.

However, it needs to be a critical consideration for a community’s development plan that it identifies, reviews and resolves all I & I sources that are adversely affecting the wastewater collection system. This is especially important when newer wastewater collection system sewers will be flowing downstream to pre-existing pumping stations and/or wastewater treatment facilities.

Surcharged operating conditions
Surcharged operating conditions due to peak flow I & I rate event.

Once a wastewater system is compromised by a peak flow I & I adverse operating condition, it must be operated under an emergency high risk flooded/surcharged condition. Higher risks also exist for failure of critical pumping station equipment and to residences, businesses and other structural assets. There are also heavier burdens on operations staff and greater costs during these events.

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Adverse operating conditions for pumping stations as well as surcharging/flooding of upstream sanitary sewers, are what I consider an “I & I attack”. Once the exceedance of a pumping station’s normal daily design flow has been reached, connected downstream collection system users are subjected to possible property damage and flooding. This can happen even when all pumps are operating during the peak flow rate event.

Many systems are seeing daily wastewater flow volumes and pump run times increasing 200% – 300 % above normal dry weather design specifications. Often, new municipal development is approved upstream of these affected pumping stations, because capacity is based mainly on recorded average annual wastewater flow values for the entire system.

However, an average annual daily flow value is always lower than what is occurring over the course of peak I & I daily flow volumes. Though much shorter in duration compared to annual daily average flows, these create immediate and adverse effects.

Therefore, I believe that peak I&I flow rates must become a critical monitoring consideration for any wastewater collection system service area and any connected wastewater pumping stations. In addition to the adverse effects, there is the considerable wasted expense of treating I & I flows. This includes extra power consumption treatment chemicals, staffing labour, recovery/restoration, and any litigation related costs from peak flow I & I events.

As shown in Table 1, I estimated these savings by using a flow meter’s daily reading in a spread sheet. The long-term benefit of such savings would be the ability to add more connected sanitary services over an extended time period before requiring any wastewater infrastructure upgrade or expansion.

Table 1. Estimated treatment costs and cost savings summary.

Estimated treatment cost of actual recorded flows
YearCalc. System I & IActual Flows Annual Total m³Actual Flows Treatment Costs ( $ )System I & I Treatment Cost portion ( $ )Treatment Cost per m³ of Flow
200816.43,353,553$771,317$126,496$0.2300
200916.53,311,707$778,251$128,411$0.2350
201011.32,965,140$711,634$80,415$0.2400
201120.53,693,638$923,410$189,299$0.2500
201211.12,808,782$674,108$74,826$0.2400
201316.93,082,698$786,088$132,849$0.2550
201416.33,198,122$831,512$135,536$0.2600
201515.13,077,196$753,913$113,841$0.2450
201615.43,236,697$809,174$124,613$0.2500

Estimated treatment cost savings if peak flow I&I sources removed
YearActual System Daily Min. m³/dayEst. System DWF for the Year m³Estimated DWF Cost Savings $Diff. Between the Actual Flows & Est. DWF m³
20086,9672,549,922$184,835803,631
20096,8532,501,345$190,435810,362
20106,4782,364,470$144,161600,670
20116,7042,446,960$311,6701,246,678
20125,9242,168,184$153,744640,598
20137,0202,562,300$132,701520,398
20146,3532,318,845$228,612879,277
20155,5612,029,765$256,6211,047,431
20166,7392,466,474$192,556770,223

Summary Review Notes:
1. After a review of the actual annual flow data, there could be a predictable annual wastewater treatment cost savings, if a wastewater collection system infiltration source reduction program was reviewed, planned and implemented over a set amount of time.
2. There would also be a beneficial cost savings (hydro usage) in the form of less daily pump run times of wastewater volumes for the wastewater system.
3. An extended increase in existing system capacity for new development is also possible by reducing system infiltration sources. If reductions in the existing wastewater system’s litre/capita/day system rates were made , an additional litre/day growth capacity would be available.

All I & I sources should be a high priority for identifying and successfully remediating, in an effort to prevent more intensified adverse effects to the collection system. Simple spot checks by operational staff of sanitary sewer manholes in a service area upstream of a pumping station can begin the visual identification of I & I compromised sections.

Once an area or section is positively identified by field staff for excessive infiltration, annual I & I reduction financial resources can be budgeted to perform detailed video investigations and plan proper resolution and repairs.

The direct effects of a collection system or pumping station compromised by peak flow I & I events resulting in flooding, are loss of public confidence, higher property insurance coverage costs, flooding lawsuits and litigation costs. These can be reduced or even eliminated by continually monitoring of pumping station flows and resolving I & I sources.

Robert Budway is with the Ontario Clean Water Agency – Essex Hub. This article appears in ES&E Magazine’s June 2019 issue.

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