Rainwater harvesting/Rainwater audit

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The first step for a designer planning a rainwater harvesting system is to educate clients and establish target goals for the rainwater collection system.[1] A water audit can accomplish both these goals at once and can serve as useful benchmarks and measurables for determining the efficacy of the rainwater harvesting system after implementation.

A water audit will separate potable from non-potable water uses and determine the appropriate level of filtration and treatment processes for each type of use. The goal is to treat water according to its end-use and ensure compliance with applicable codes, laws and regulations. Drinking water requires more treatment and attention, than say toilet water flushing or certain kinds of irrigation. Unnecessary treatment devices can be a waste of money, resources and time on behalf of the owner or manager of the system.

For commercial, institutional and industrial users, a facilities or building survey, such as the one produced by the New Mexico Office of the State Engineer, can provide a helpful overview of the process.[2] Namely, the purpose of this exercise is to understand where and how much water enters the system and where and how much water leaves the system.

For larger facilities, relevant information can be collected from company and utility records. People familiar with the systems, such as operations and maintenance supervisors and staff can be of great help.

The following information may be helpful:

Step 1: Information Gathering from Records and Plans

  • physical size of facilities, floor plans, plumbing, irrigation and civil schematics and drawings
  • total number of occupants and/or employees and average number of people for certain periods of the day
  • identifying water meters
  • inventory of water consuming fixtures and features
  • inventory of all water-using equipment with manufacturers' flow rates
  • outdoor water use rates, schedules and volumes
  • utility records (water and sewer) from previous two years
  • records for real water use, defined as readings from meters and submeters, water wells and water tank deliveries
  • prior water audits
  • anticipated water and sewer billing rates for next two years (from the utility)
  • calculate total daily water usage

Step 2: Onsite Survey

  • facility walk-through with occupants, supervisors and staff if possible
  • identify and list all equipment that use water, including water-using process equipment, cooling towers, boilers, faucets, showers, sinks, toilets, water fountains, etc.
  • compare against previously collected information and reconcile discrepancies
  • note devices that use water for more than one kind of operation (e.g. ice maker for drinking and cooling)
  • calibrate water meters for accuracy
  • measure amount of water used by equipment and compare against manufacturers' descriptions
  • ask people familiar with system for water conservation suggestions with each water-use process
  • measure water quality at different end-use points for consideration in reuse in other processes (e.g. chemical make-up, pH level, conductivity, total dissolved solids (TDS/ppm), waste content, and temperature).
  • measure exterior water use
  • calculate total daily water usage and compare against consumption data from previous figures

Using this information create a report for yourself and the clients.

Step 3: Prepare an Audit Report

  • put together an up to date package of facility diagrams, blueprints and water flow charts
  • current list of all water-consuming devices with manufacturers' flow rate compared to measured flow rates
  • occupancy rates and schedules for people and equipment use
  • month-by-month landscape irrigation watering schedule
  • water flow chart from entering the system to its eventual points of final discharge
  • additional water-use observations revealed by walk-through audit and analysis
  • evaluation of financial cost of water for entire building/facility

Step 4: Determine Total Water Cost

  • utility bill cost per unit volume of water
  • water heating costs
  • cooling water costs
  • energy cost of pumping water from wells or onsite for pretreatment, including filtering, purifying, softening and disinfecting
  • chemical treatment costs, including boilers and cooling water towers
  • any predisposal treatments
  • disposal costs of hazardous aqueous substances
  • stormwater management costs, including typical fines, or expenses related to site stormwater characteristics
  • sewer discharge costs, which can include amount of water discharged, total dissolved solids, and other water-quality considerations

This is a step that helps not only to plan and integrate a rainwater harvesting system into an existing network of water supply and consumption, but also to determine other, just as critical, improvements that can help avoid the need for extra water in the first place.


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  1. Celeste Allen Novak, et al. Designing Rainwater Harvesting Systems : Integrating Rainwater into Building Systems. Hoboken, New Jersey, Wiley, 2014, p. 160.
  2. New Mexico Office of the State Engineer. A Water Conservation Guide for Commercial, Institutional and Industrial Users. July 1999, P. 18.