Water and Pollution Control Department

The City of Ames Water and Pollution Control Department prepared a water supply demand report in August of 2008.  The report was developed to provide the Departmental view on the future growth in water demand for the City for use in determining a timeline for capacity expansions and sizing guidance for such expansions. 

One key finding discussed in the report was that significant increases in water demand in the last two years (2006 and 2007) were largely a reflection of metering errors rather than real growth in demands.  This finding reduces the probability that the water demands of the City will exceed current treatment capacity before planning and development of additional capacity can be completed under a reasonable schedule.

The report describes three different methods of projecting future water demands for the City:

    1) projections of peak demands based on most recent 5-year and 10-year demand trends,

    2) projections using population projections from the City's Land Use Policy Plan (LUPP) and per capita demand data from historical records, and

    3) projections using average day demand and applying historical peaking factors to arrive at peak demands.

The conclusion reached was that the average day projections should be used along with peaking factors and a reserve for industrial growth to achieve a plant peak 3-day design capacity of 16 MGD for a design year of 2033.  Part of this projection was an assumption that reserve capacity should be included in the design capacity to allow for planning and execution of additional plant expansions at the end of the design life.  Thus the actual projected peak 3-day demand of 13.6 MGD, which included the industrial reserve of 0.5 MGD, was then divided by 85% to arrive at the 16 MGD design capacity.

In an effort to confirm or suggest revisions to the department's projections, the consulting team conducted a demand projection of their own.  Starting with a review of historical population and water demand data and the population projections contained in the city's Land Use Policy Plan (LUPP), the team first considered the overall patterns from the historical data in an attempt to rationalize a basis for future demand projects.  Basically two methods for projecting future water demands were considered, similar to but not identical to the methods described in the Department report.

The first method considered is typical of the water utility industry in that it utilized historical records of peak daily demands on a per capita basis and then applied those to population projections.  Figure 3.1.1. shows the historical record of per capita water demands for peak day, peak 3-days, peak month and average day for the past 30 years and projected these forward in a linear fashion to the targeted design date of 2033.

It is of interest to note that peak day and peak 3-day data show a relatively consistent trend over the record period while both maximum month and average day show a long term trending downward.  In discussions with Department staff, a number of possible explanations were offered for the downward trend in average per capita use including; water conservation efforts by Iowa State University, a major water user in the system, removal of the City's Power Plant cooling water use from the treated water demand in the mid 1990s, and removal of irrigation water demand from the City's Homewood Golf Course in 2001 as they began to use their own well for such water.  Among other factors that may contribute to a long term trend downward in average per capita demand are the changes in the plumbing code adopted in 1994 requiring low use water fixtures in new and remodeled homes.  We would expect that peak rates are more highly influenced by domestic and commercial irrigation demands, and thus less susceptible to influence by the major non-domestic users.

Before applying these per capita rates to population projections to arrive at future system demand projections, we felt it was useful to consider normal statistical variation in the various rates.  Figures 3.1.2. through 3.1.5. show the individual per capita use rates along with upper and lower control limits based on average rates plus or minus three times the standard deviation of the data sets (3-Sigma).

Because it is necessary, or at least highly desirable, to ensure the ability to supply the peak demands of the system users, we believe that the upper control limits for the various per capita use records should be used in sizing future treatment unit capacities.  Based on this assumption we are suggesting that a peak day use of 200 gallons per day per capita be used in projecting future maximum peak day demand rates.  Use of this number will provide a high level of confidence that the actual peak day per capita demand will not exceed the projection except under very special and un-predictable conditions.  Similarly, peak 3-day, maximum month and average day per capita demand rates should use 194 gpcpd, 155.5 gpcpd and 137.6 gpcpd rates respectively.

Once per capita rates have been determined, it remains to apply these to projected populations for the design planning date designated for this plan of 2033.  For this we have accepted the LUPP projections for maximum and target populations for the City with the exception that we adjusted the numbers in the plan to compensate for an apparent typographical error in the rate of growth between 2020 and 2030.  We believe that the intent of the plan was to continue the rate of increase shown in the period of 2010 to 2020 for the last ten year period of the planning projection but the plan actually shows a rate one-half of this rate in terms of population growth per year.  After adjusting for this and extrapolating to the year 2033, we arrived at a projected populations of 64,800 for the high projection, 62,800 for the target projection and 54,820 for the low end of the envelope.

Applying a peak day per capita demand rate of 200 gpcpd to these figures, we arrive at a design size for peak day capacity ranging from 13.0 MGD for the high population growth projection, 12.6 MGD for the target rate projection and 11.0 MGD for the low growth projection.  If the adjustments utilized in the Department demand projections are applied, i.e. a 0.5 MGD reserve capacity for new industrial trowth and targeting an actual demand of only 85% of actual plant capacity at the end of the 20 year design life, the peak day design capacity becomes 15.8 MGD for the maximum population projection, the target growth rate design number if 15.4 MGD and at the low growth rate number is 13.5 MGD.

The second method utilized for projecting peak day demand at the design year was simple extrapolation of the total peak day demand rates from historical records.  For this option, we determined the linear trend line rate of growth for the average peak day based on the most recent 30 years of historical data.  That trend line was extended to the design year and upper and lower control limits were applied to this line based on the standard deviation of the data set.  This option did not consider population projections separately but only assumes that increased peaking rates represent increasing population growth along with any changes in use patterns among the customer base of the utility.  Figure 3.1.6. shows the Peak Day Demand trend line rate with control limits based on both 2-Sigma and 3-Sigma control limits.  Figure 3.1.7. shows a  similar graph for the peak 3-Day Demand rate.

As indicated on these figures, the peak day and peak 3-day design projections for the design year are essentially the same at 11.59 MGD and 11.51 MGD using the 3-sigma upper control limits.  Again, using the 0.5 MGD industrial allowance and the 85% factor of safety to allow for planning and construction of further expansion at the end of the design life, the design capacity would be 14.22 MGD and 14.13 MGD for peak day and peak 3-day designs.

Using these methods, and following the suggestions in the Departmental report for industrial growth allowance and the 85% factor, we would seem to be finding a necessary design capacity for the year 2033 in the range of 15 to 16 MGD when rounding up to the next higher even million gallon sizing configuration.  We do find the suggestion of 85% factor somewhat unusual in our experience.  If the intent of this factor is to provide time for planning and implementation of further expansions or replacement at the end of design life, then it would seem that use of a percentage based on the expected annual growth in demand and the time required to accomplish the planning and construction efforts would be more readily justifiable.  In this instance, the annual growth rate provided in the LUPP (adjusted for the error we believe occurred in that document) is about 0.65% per year.  Assuming that five years would be needed to accomplish planning and construction of any needed expansions at that point in time, the expected growth in demand during those five yeras would be about 3.5%.  From this, it would seem that increasing the projected design deman rate by 3.5% would serve the purpose posed for the 85% factor.

At the same time, we have noted at least two recent instances in Iowa where high tech industry has proposed to move facilities into Iowa communities and have a need for significant amounts of cooling water for computer cooling (1 to 1 1/2 MGD in peak summer periods).  This is happening in Council Bluffs with Google and West Des Moines with Microsoft.  These instances point to a new type of 'wet' industry that differs from what we have typically envisioned, such as food or chemical processing companies with lower paying jobs and significant wastewater loading.  Having the ability to support the water needs of one of these high tech industries would seem to fit into the categories of industrial growth often cited as desired by the Ames community.  Target industry have been described locally as those providing high tech jobs for our college graduates and those industries not likely to result in significant demands on our wastewater facilities or other community environmental systems.  Considering this, it may be appropriate to provide additional industrial capacity in any major expansion or replacement of the water system.  It is well recognized that the marginal cost of providing additional capacity in a project is significantly less than the cost of adding that capacity at some future time as a separate project.

If we suggest providing the high or target growth demand for the design year of 2033, and add a full one million gallons of speculative industrial capacity and then increase this design number by 3.5%, the design capacity becomes about 14.5 MGD for the peak day design under high growth projections and 14.1 MGD for target growth conditions.  Similarly, if we use the peak day trend projections and add the industrial capacity and planning time factor of 3.5%, the peak day design value is found to be 13.03 MGD.  The key finding here is that planning for additional capacity in excess of current plant capacity is certainly appropriate.  Because, as we explained above, marginal costs for capacity are low compared to overall project costs, these numbers should be considered minimum plant design targets with actual design capacities at or near the 16 MGD suggested in the Department Staff report very justifiable.  

WATER QUALITY GOALS

In any discussion of water treatment facilities, the ability of those facilities to consistently meet required and desirable standards of water quality is key to a complete evaluation.  The following discussion treats both current and future compliance of the Ames water supply with Federal and State Drinking Water Standards and also addresses secondary standards and aesthetics. 

In addition to the quantities of water required, the quality of water must be considered in order to assure that the water supplied is acceptable to both consumers and the regulatory bodies.  Regulatory requirements are established under the Federal "Safe Drinking Water Act" which sets maximum allowable levels of various substances that may be found in water supplies.  These standards are referred to as "Primary" standards and "Secondary" standards.  Primary standards are required and secondary standards are only recommended. All of the standards are under regular review.

Some substances, while not appearing on either the Primary or Secondary Drinking Water Standards, are still important to water users.  Most notable among these are hardness and alkalinity.

Based on information available from the existing well sources from which Ames draws its water, the raw water sources prior to any treatment meet all of the primary drinking water standards and most of the secondary standards.

Present treatment facilities provide for removal of dissolved gases such as hydrogen sulfide that may cause odors, for aeration of the water to allow the iron and manganese to be oxidized for removal on the filters, and for softening of the water by lime softening processes.  Chemical addition is also practiced at the water plant where chlorine is added to provide for residual disinfection capability in the distribution system, fluoride is added for dental health purposes and polyphosphates are added to stabilize the water and reduce any tendency for disposition or corrosion.  These processes, along with the initial high quality of the raw water supply, allow the finished water to comply with all primary drinking water standards and all secondary standards other than the recommended pH range which is exceeded by the Ames finished water due to the increases in pH caused by the lime softening process.

The following table lists various components of the Federal Safe Drinking Water Act (SDWA) and the City's current situation relative to compliance with the Act's various requirements.

Note regarding ground water rule:  The ground water rule is expected to require that a ground water system that has been identified as having significant deficiencies must do one ore more of the following: eliminate the source of contamination, correct the significant deficiency, provide an alternate source water, or provide a treatment which reliably achieves at least 99.99 percent (4-log) inactivation or removal of viruses before or at the first customer.  While there is some question whether the current treatment system in Ames provides for 4-log inactivation of viruses due to the use of chloramines for disinfection, it is also unlikely that the City will be found to have significant deficiencies with regard to the protection of their groundwater sources in terms of protecting those sources from possible contamination from fecal coliform and virus sources.  should the City find they need to meet the 4-log inactivation requirement for viruses in the future, it is likely that a change in disinfection practices, such as use of chlorine dioxide, will allow them to do so.

Regardless of whether or not a recommendation is made to upgrade or expand the existing treatment facilities or to replace the plant with a new plant of similar or different treatment technologies, the minimal goals of any such project would have to be the meeting of Primary Drinking Water Standards.  Since current raw water quality will essentially meet all primary drinking water standards, treatment technology selection will not likely be heavily influenced by these standards.  Since compliance with secondary standards is optional for water supply utilities, it is up to the City to determine what level of compliance with these voluntary criteria is necessary to best serve the utility's customers.  Similarly, it is within the utility's authority to choose to continue to provide softening at the municipal level for hardness reduction.  Having said that, the City of Ames has publically stated their commitment to providing the highest quality of water to their customers consistent with affordable water rates.  It seems readily apparent that in meeting this commitment and the expectations of the utility's customer base, the City will choose to continue to provide for iron and manganese removal, taste and odor control through removal of hydrogen sulfide, fluoride addition, and softening to some level in the 9 to 11 grain per million range.  In other words, the finished water quality goal of any project considered in this study would be something very close to the current finished water quality being produced by the Department.

Based on current water quality and national and state water quality standards, we do not anticipate any future water quality issues that are not already being addressed by the City other than the remote possibility that the City may have to provide a 4-log inactivation of viruses in their treatment processes.  As mentioned above, this can likely be accomplished by some relatively straightforward changes to the disinfection practices in the city.

Current treatment meets all quality goals for the City and this situation is not anticipated to change in the foreseeable future due to any known changes in customer demands or Federal or State regulation changes.  Should a decision be made to expand existing facilities or replace them, technologies should be selected based on their ability to closely match current finished water quality, i.e. low iron, manganese, and hydrogen sulfide for aesthetic reasons, and a moderately soft water.  If it is possible to continue to utilize chloramines as the primary disinfectant, that will likely also contribute to maintenance of the current taste and odor (or lack thereof) of the finished water, one that is appreciated locally and has even won national acclaim.