A water utility faces many challenges when planning water, wastewater, and reclaimed water service for a developing area.  The management of reclaimed water can pose a particular challenge.  Unlike in the past, when reclaimed water management was referred to as effluent "disposal," reclaimed water is now considered a valuable resource.  Water utilities are aggressively expanding their reuse irrigation systems to offset demands on potable water for irrigation and extend the sustainability of their water supply sources.  Their challenge in doing this comes with the need to adequately plan for dry and wet weather events.

During dry weather, reuse demands can far exceed supplies.  This situation may require a utility to have either a supplemental supply source to augment their reuse system, such as stormwater or groundwater, or large amounts of storage.

Four Step Process

Conversely, during wet weather, reuse demands may fall well below the supply of reclaimed water.  Water utilities are required by state regulation, however, to have suitable back-up wet weather management options, such as storage, rapid infiltration basins (aquifer recharge), aquifer storage and recovery, deep aquifer injection, or wetland hydration.

Projecting the magnitude and frequency of supplemental supplies and wet weather
management needs is crucial to the successful operation of a water reclamation facility. Estimates of these requirements can be developed using supply and demand modeling methods.  The four steps involved in this process are:

Data Collection.  Required data could include, but are not limited to:

• Rainfall dates and amounts
• Historic reclaimed water generation
• Historic reuse irrigation demands, application areas, and application rates
• Historic wet weather reclaimed water management flows
• Existing storage facility capacity
• Local evapotranspiration and soil types.

Annual Average Supply and Demand Development.  Estimates of current average daily flows for reclaimed water supply, reuse irrigation, and other reclaimed water management options can be developed based on the recent flow data collected.  Projected (future) annual average daily supply and demand flows, which are also needed for supply and demand modeling, may be available from the water utility or they can be developed through various techniques based on historic flows and historic and projected populations and land uses.  For example:

• Future reclaimed water supply generation could be linearly projected to estimate future annual average supplies.
• Future reuse irrigation demands can be estimated from projected land uses and characteristic reclaimed water application areas and rates for similar land uses.

Representative Series Development.  Long-term representative series of reclaimed water flow and reuse demand can be developed to evaluate reclaimed water alternatives and develop estimates of supplemental supply and wet weather management.

These series can be developed using continuous simulation (analytical) models that allow for the current variability characteristics to be preserved under a wide range of conditions.  This approach meets the requirements of Florida Department of Environmental Protection (FDEP) rules 62-610.464(2) and 62-610.414(2) F.A.C. regarding storage requirements for slow-rate land application systems.  In particular, regulations 62-610.414(2)(a), (b) and (d) call for:

• System storage ponds to have capacity sufficient to retain the reclaimed water under adverse weather conditions.
• The system storage period to be based on the volume of storage that would be required for a ten-year recurrence interval using weather data from or representative of the area involved.
• Analytical means to be used to determine system storage requirements.  These means must also account for all water inputs into the system.  They can be water balance calculations or computer hydrological programs, such as FDEP's LANDAP program.

Reclaimed Water Flow.  Reclaimed water flow can be estimated by developing an analytical or statistical model based on historic reclaimed water supply flows, including linear growth, seasonality and rain response.  Analytical and statistical methods available to develop representative series include:

• Averaging techniques (moving average, average Julian day, day-of-week average, etc.)
• Evaluation of antecedent precipitation indices
• Cross-correlation (with rainfall or other parameters as necessary)
• Auto-correlation.

The intent is to develop a series that adequately represents typical conditions experienced at the water reclamation facility in the past.  The analytical or statistical methods most appropriate to characterize a series of data can vary between different data sets from different facilities.

Different combinations of methods can be applied until an adequate synthetic series is developed.  Current and future representative flow series can then be calculated by normalizing the developed representative flow series and multiplying it by the average current and future reclaimed water flows, respectively.

Reuse Irrigation Demand.  Reuse irrigation demand can be simulated by developing a statistical model incorporating theoretical reuse demand (such as AFSIRS or Blaney-Criddle models), seasonality, and rain response.  Representative irrigation demand series can also be developed based on historic reuse irrigation demands using similar analytical or statistical methods as discussed for reclaimed water supply.  Current and future representative demands can then be normalized and multiplied by the current and future average reuse demand, respectively.

Wet Weather Management Options.  Analytical methods to represent existing wet weather management options can be developed using a variety of techniques.  For example, rapid infiltration basins (RIBs) can be modeled as a linear reservoir within the surficial aquifer.  The rate of discharge from the surficial aquifer is generally proportional to the volume of water stored in it and can be estimated from historic RIB operational data.

Supply and Demand Model Development

The supply and demand model (Figure 1) is based on the flow series and analytical techniques developed to represent reclaimed water supply generation, reuse irrigation demands, and wet weather management.  The model itself can be created using standard spreadsheet software or system modeling software.

The amount of reclaimed water that a water reclamation facility generates and the demands placed on the plant's reuse system can be modeled on a daily, weekly, or monthly basis.  If the sum of demands exceeds available supplies for a given time period (n), then supplemental supply is needed.  If supply exceeds the sum of demands for a given period (n), then wet weather management is needed.  The conceptual equation that the supply and demand model solves is:

Supplemental Supply (-) or   =  Supply_n  -  ΣDemands_n
Wet Weather Need (+)n

Estimated wet weather management can be compared to the available capacity of existing wet weather management and storage facilities.  An example of problem set up, results, and predicted flow are illustrated in Figure 2.

Hypothetical Examples

Dry Weather.  If a water reclamation facility is projected to produce 10 mgd (38 Ml/d) annual average daily flow and have 5 mgd (19 Ml/d) average annual daily flow for reuse irrigation customers and 5 mgd (19 Ml/d) average annual daily flow in RIBs for reclaimed water management, the overall system is balanced on an annual average basis.  During a period of prolonged dry weather, however, reclaimed water supply can be 60 percent to 70 percent below annual average due to reduced inflow and infiltration into the wastewater collection system.  At the same time, reuse irrigation demands can be up to three times higher than average.  Supply on a given day may drop to 6 mgd (23 Ml/d) while demands could be as high as 15 mgd (57 Ml/d).

This scenario results in a 9 mgd (34 Ml/d) supplemental supply need on that day and an overall supplemental supply need for the year even though the system is balanced annually.

Wet Weather.  During wet weather, reclaimed water supply can be up to twice the annual average and irrigation demands can drop well below that average, say to 25 percent or less of annual average or below.  During prolonged wet weather events when rainfall is above average for weeks or months, irrigation demands can fall close to zero.  For the same hypothetical 10 mgd (38 Ml/d) facility discussed above, supply during wet weather could be 20 mgd (76 Ml/d) while irrigation demands could be as low as 1 mgd (4 Ml/d).  This results in a 19 mgd (72 Ml/d) wet weather management need on a given day; but the facility has only 5 mgd (19 Ml/d) in RIB capacity.

Though RIBs can accommodate flows in excess of their annual average capacity for limited periods, extended periods of above-average loading and rainfall can result in the surficial aquifer soil matrix beneath the RIBs reaching saturation and potentially causing ponding or flooding.  These seasonal periods can result in an annual average wet weather need that is higher than the wet weather need estimated based on an annual average water balance.

Conclusions

Seasonal effects can result in supplemental supply and wet weather management requirements not revealed by an annual average water balance evaluation.  Development of a model that considers the variability exhibited by typical reclaimed water supply and demand flows can assist a water utility in future planning and facility operations.  Supply and demand models can also be applied to other systems, such as water supply systems, and to natural systems, such as lakes or wetlands.