Checklist for Review and Optimization of Treatment for Protection Against Waterborne Disease

View inside a treatment plantThe key barriers used in water treatment to prevent the passage of pathogens from raw water to finished drinking water are:

  • Source protection
  • Coagulation
  • Flocculation
  • Sedimentation
  • Filtration
  • Disinfection
    When a treatment plant is optimized, careful attention must be paid to both physical removal and disinfection. It is also important when making an assessment to look at the source of supply, considering potential contamination sources, and evaluating possible source protection/management strategies and monitoring practices.

    The following information is intended to aid supply source and treatment plant facilities in evaluating their operational effectiveness for protection against microbial contamination and waterborne disease.

    Please consult the public water systems page for additional information related to monitoring, reporting and engineering.

    Water Supply Source

    Combined sewer overflowSource Protection Measures

    1. Assess watershed/recharge area impacting supply for potential problem sources, consider:
      • Agricultural runoff
      • Combined sewer discharges
      • Sanitary sewer overflows
      • Wastewater treatment plant discharges
      • Septic tank systems
      • Livestock operations, such as dairy farms, feed operations, grazing
    2. Develop and implement source protection/management strategies, consider:
      • Control livestock access to the source and provide control and treatment of runoff from livestock areas
      • Wastewater discharges, including outfall location, discharge standards and permit limits of treatment facilities, installation, design standards and inspection of on-site wastewater disposal systems
      • Management of recreation sites
      • Community development considerations, such as land use controls, zoning and density standards and direct land ownership by water system
      • Agricultural/forestry practices, such as stream buffer strips, soil erosion control methods and runoff control measures

    Source Monitoring

    1. Evaluate source quality monitoring measures:
      • Is routine raw water quality parameter testing done on a frequent enough basis to affect required changes in chemical feed rates in a timely fashion?
      • Are continuous online monitors used for water quality monitoring? Especially for run of river supply, consider continuous, online turbidimeter to monitor changes in water quality. Implementation of an early monitoring/warning system upstream on the source of supply may be beneficial.
      • If continuous, online water quality monitors are used, are they calibrated on a routine basis in accordance with manufacturer's recommendations?
      • Are operators trained to be observant to any non-routine changes in water quality, such as odors, color, presence of algae, etc., and are procedures in place to respond to changes?
    2. Evaluate plant flow monitoring measures:
      • Are plant flow monitoring devices provided and installed for accurate metering of plant flow rates?
      • If flow is split within the plant, are adequate flow measurement and control provisions provided?
      • Are flow meter maintenance and calibrations performed on a routine basis in accordance with the manufacturer's recommendation?

    Rapid Mixing - Coagulation

    water treatment plantChemical Addition/Mixing

    1. Evaluate points of chemical addition, consider:
      • Is adequate mixing provided for chemical dispersion?
      • Could improved mixing be obtained by adjusting point(s) of chemical addition (i.e., chemical application at point of turn or tee for increased mixing or better dispersion through use of diffuser or directing chemical discharge near mixer impeller)?
      • Are points of chemical application located for most effective utilization of chemical or could enhancements be made by adjustment of feed points (i.e., coagulant aid polymer addition applied further downstream of coagulation addition)?
    2. Evaluate rapid mixing:
      • Are adequate mixing provisions provided for immediate dispersion of the applied chemicals?
      • Are mixing characteristics of rapid mix equipment known and is sufficient mixing energy provided?
      • Are there provisions for adjustment of rapid mix speed to vary mixing intensity?
    3. Assess chemical feed equipment and rate adjustment practices:
      • Is standby equipment provided to assure chemical feed capability?
      • Are clear procedures established for chemical feeds based on water quality conditions?
      • Are flow paced controls provided for adjustment of chemical feed rates when plant flow rate changes are made?
      • Is chemical feed equipment checked/calibrated on a routine basis?
      • Are there provisions to readily determine the amount of chemical fed (i.e., scales, level gauges, etc.)?

    Coagulation Considerations

    1. Assess coagulant addition practices (for optimum particle removal), consider the following:
      • Are procedures/practices in place for determining optimum chemical dosage (i.e., consider jar testing procedures, streaming current detectors and zeta potential meters as operational tools)?
      • Do optimum coagulant conditions (proper pH, adequate alkalinity level) exist for coagulant used? If not, are chemical storage and feed systems available to make adjustments?
    2. Evaluate use of coagulant aid for enhanced treatment capability.
    3. Consider use of alternate coagulants and evaluate treatment effectiveness for particle removal.
    4. Maintain proper coagulant control procedures, consider the following:
      • Establish and maintain well-organized records of plant operations and performance under the varying raw water conditions encountered.
      • Routine monitoring of settled water turbidity for process control purposes.
      • Installation and use of coagulant control devices such as a pilot filter, streaming current detector or zeta potential monitoring.


    1. flocculationEvaluate flocculation basins for optimum flow characteristics, consider the following:
      • Do inlet conditions provide good flow distribution to multiple basins and within individual basins themselves?
      • Are basins baffled, or could they be baffled to improve flow distribution at inlet, exit and between stages?
    2. Assess basin detention time for adequate reaction time (20 to 40 minutes at maximum flow rate is typically desirable), consider use of tracer study to evaluate.
    3. Evaluate flocculation equipment, consider the following:
      • Assess operating characteristic of existing flocculation equipment and establish level of mixing that can be provided at the range of operating temperatures encountered.
      • Is flocculation equipment designed for tapered flocculation and operated as such?
      • Are there provisions for adjustment of flocculation equipment speed to vary mixing intensity?


    1. Well building in waterEvaluate basin entry and exit conditions, consider the following:
      • Review inlet conditions for proper flow distribution among multiple basins and within basins themselves.
      • Is inlet to the basin baffled or would addition of target or perforated baffle improve flow distribution across basin?
      • Are effluent weirs provided and weir loading rates acceptable, or could additions such as weirs or some type of effluent baffle improve process operation?
      • Is there sludge carry-over, either continuously or on an occasional basis, and could improvements be made to alleviate carry-over?
    2. Evaluate basin residence time characteristics, consider use of tracer study to evaluate.
    3. Evaluate clarification rates for the range of flows typically encountered under current conditions of operations to determine if rates are acceptable.
      • If rates are marginal, consider addition of tube/plate settlers.
    4. Evaluate sludge draw-off capabilities:
      • Check draw-off procedures to assure that sludge levels are properly maintained at low levels so sweep up conditions do not occur.
      • Is sludge draw-off/equipment maintained in good operational condition?
      • Are procedures in place for proper manual cleaning to avoid excessive applied water turbidity.
      • Is continuous sludge collection equipment provided and maintained in good operational condition? If not, consider addition.
    5. Consider use of the filterability index testing as one measure of coagulation effectiveness.


    FiltrationFilter Operational Considerations

    1. Set operating goal of 0.1 NTU to initiate filter backwashing.
    2. Assess filter flow rate operations for potential problems with rapid filtration rate changes, consider the following:
      • Do all operators understand importance of gradual rate changes to minimize impacts of rate changes?
      • Do rapid changes occur due to method/manner of increase in total plant flow?
      • If automatic flow control scheme is provided, is system operation stable and rate changes made in a smooth manner without hunting?
      • Are rate control valves and instrumentation maintained in proper operating condition so rate transitions are achieved slowly and smoothly without hunting?
      • Do rapid rate changes occur as a result of filter backwashing procedures?
    3. Minimize water quality upsets following filter backwashing, consider the following options:
      • Plant flow conditions permitting, operate with last filter backwashed remaining off-line to allow media to settle until another filter requires backwashing and is removed from service.
      • Provide/utilize filter-to-waste provisions for a sufficient time interval following backwashing.
      • Gradually increase the filtration rate to normal rate following backwashing over a sufficient time interval.
      • Do not place several filters on line at the same time.
      • Use of polymer or coagulant addition to the filter backwash at the end of filter backwashing period.
    4. Assess use of a filter aid, particularly at times of rapid changes in raw water conditions, consider:
      • Filter aid polymers
      • Small dosage of plant coagulant chemical added to influent of the filters.
    5. Evaluate recycling of waste filter washwater, consider the following:
      • If present, microbial contaminants will concentrate in waste filter backwash stream and recycle should be discontinued.
      • Recycle rate should not exceed 10 percent of the plant flow rate.
      • Treatment to remove particulates should be considered prior to return (i.e. clarification step)
      • Provisions should be made for alternate disposal of filter backwash.
    6. Establish/maintain operating policy to backwash any filter removed from operations prior to placing it back in service.

    Filter Media Evaluation

    1. Routinely inspect the condition of the media.
      • Drain filter periodically and check the surface condition. Surface should appear smooth and even. If cracks, ridges or mud balls exist, it indicates problems with filter backwashing. To check for mud balls, rough up the first couple inches of the media surface and examine the media for mud ball formations.
      • Viewing media grains under a low power microscope can give a good idea of condition of media. Media should be viewed to see that grain size and shape remain relatively constant. (Keep sample of original media for comparison purposes.)
      • Probe bed periodically with metal rod to determine location of support gravel.
    2. Routinely monitor media depth to determine if media loss or growth has occurred.
    3. Periodically sample media and perform sieve analysis to determine effective size (d10), uniformity coefficient (d60/d10) and d90 size for backwash considerations.

    Filter Instrumentation Considerations

    1. Evaluate effectiveness and/or level of monitoring instrumentation for filter operation, consider the following:
      • Installation of continuous online turbidimeters for process control purposes.
      • Consider use of particle counters to optimize particle removal.
    2. Evaluate/consider use of flow control scheme for filter flow splitting purposes.
      • Are flow transitions made slowly and smoothly?
      • If not provided, would an automatic flow splitting control scheme aid in making flow transition in a smoother fashion?
    3. If not provided, consider use of automatically controlled backwash procedure for more consistency in filter backwashing.
    4. Confirm that instrumentation for filter monitoring and control is calibrated and maintained in accordance with manufacturer's requirements.

    Filter Backwash Consideration

    1. Evaluate filter backwashing capabilities and procedures for effective operation, consider the following:
      • Are provisions made for auxiliary scour of the filter media, i.e., surface wash or air scour capabilities?
      • Are backwashing provisions sufficient to provide adequate wash rates for media fluidization and for a sufficient wash duration?
      • Are proper backwashing procedures and the importance of effective backwashing understood by all operators?
      • Have clear operating criteria been established for initiating filter backwashing (time, head loss, turbidity, particle counts)?
      • Is the general appearance during backwashing such that media is uniform with a lateral motion on the surface? If violent boils are noticeable, problems with clogging or gravel displacement exist. Also check for and eliminate causes of undesirable bursts of air, which could upset media stratification.
      • Does relative distribution of washwater appear uniform in which washwater clears up over the entire surface at the end of the wash? If areas clear up more slowly, it can indicate that clogging of the media, support gravel or underdrain orifices is taking place.
    2. Periodically probe filter bed during backwashing to determine if media is properly fluidized or if there are hard unfluidized areas in the bed.
    3. Evaluate/observe degree of media expansion and determine if sufficient clearances are provided between the top of the media and filter backwash troughs to prevent excess loss of media.


    1. Mouse crawling into a wellheadEvaluate disinfection feed system capabilities and monitoring and control features, consider:
      • Does feed equipment have adequate capacity to provide a sufficient concentration of disinfectant during times of peak flow and disinfectant demand?
      • Is feed equipment properly maintained and in good working order for dependable operation?
      • Is redundant feed equipment provided to assure disinfectant feed capability?
      • Consider use of continuous residual analyzers for monitoring disinfectant residual tied to alarm if residual level drops too low. Consider continuous analyzer short distance downstream of disinfectant addition for process control/feed verification purposes.
    2. Evaluate disinfectant points of application, consider:
      • Is disinfectant applied with thorough and quick mixing?
      • Are disinfectants added at location where they will be compatible with purposes of entire process train (i.e., adding free chlorine and powdered activated carbon close together may result in inefficient use of both chemicals)?
    3. Evaluate disinfectant contact time, consider the following:
      • Estimate can be made using baffling factors, but more exact numbers can be obtained through use of tracer studies.
      • Consider impacts of changing water level in clearwell storage.