Surface Water Treatment Process Control & Troubleshooting
Learn how operators control and troubleshoot surface water treatment by evaluating raw-water changes, coagulation, flocculation, clarification, filtration, disinfection, process trends, alarms, and treatment interactions.
Surface water treatment requires continuous process control because raw-water quality can change quickly. Rainfall, runoff, algae, temperature, turbidity, organic matter, and source-water conditions can all change the amount and type of treatment required.
Good troubleshooting begins by looking at the entire treatment train. A problem observed at the filters may begin with coagulation. A disinfection problem may be related to upstream particle removal. A sudden instrument alarm may represent either a real process upset or an instrument problem. Operators should use measurements, trends, observations, and process relationships together.
Surface Water Treatment Is an Integrated Process
A conventional surface-water treatment train can include:
- raw-water intake;
- chemical addition;
- coagulation;
- flocculation;
- sedimentation or clarification;
- filtration;
- disinfection;
- finished-water storage.
Each step affects the next.
Process Control
Process control means using operating data and observations to keep treatment processes within desired conditions.
Important information can include:
- raw-water turbidity;
- pH;
- alkalinity;
- temperature;
- plant flow;
- chemical dose;
- clarified-water turbidity;
- individual filter turbidity;
- filter head loss;
- disinfectant residual.
Start with Raw Water
When treatment performance changes, first determine whether raw-water conditions changed.
Possible changes include:
- higher turbidity;
- lower turbidity;
- temperature change;
- pH change;
- alkalinity change;
- algae;
- organic matter;
- taste and odor compounds.
Storm Events
Rainfall and runoff can rapidly change source-water quality.
Storms can increase:
- turbidity;
- suspended solids;
- organic matter;
- color;
- treatment demand.
Do Not Assume Yesterday's Dose Is Correct Today
A chemical dose that produced excellent treatment under yesterday's raw-water conditions may not be appropriate after a storm or seasonal change.
Plant Flow Matters
Flow affects:
- chemical feed requirements;
- mixing conditions;
- clarifier loading;
- filter loading;
- detention time.
Chemical Feed and Flow
A common relationship is:
Chemical Feed, lb/day = Flow, MGD × Dose, mg/L × 8.34
Flow-Change Example
A plant normally treats 2.0 MGD at a coagulant dose of 20 mg/L.
Chemical Feed = 2.0 × 20 × 8.34
Chemical Feed = 333.6 lb/day
If flow increases to 3.0 MGD while the desired dose remains 20 mg/L:
Chemical Feed = 3.0 × 20 × 8.34
Chemical Feed = 500.4 lb/day
The required chemical mass increases because the amount of water being treated increased.
Verify Chemical Feed
Do not rely only on a pump setting.
Verify:
- chemical tank level;
- chemical concentration;
- feed-pump calibration;
- feed-line condition;
- injection point;
- actual feed rate.
Coagulation Control
Coagulation performance depends on factors such as:
- coagulant dose;
- pH;
- alkalinity;
- raw-water turbidity;
- organic matter;
- rapid mixing.
Signs of Poor Coagulation
Possible signs include:
- weak floc;
- small floc;
- poor settling;
- high clarified-water turbidity;
- rapid filter head-loss development;
- short filter runs.
Jar Testing
Jar testing can help operators evaluate:
- coagulant dose;
- pH adjustment;
- chemical combinations;
- floc formation;
- settling.
Jar-test results should be confirmed through controlled full-scale operation.
Flocculation Control
Flocculation should provide enough particle contact for floc growth without creating excessive shear.
Poor flocculation can result from:
- mixing that is too weak;
- mixing that is too strong;
- insufficient detention time;
- hydraulic short-circuiting;
- cold water;
- poor upstream coagulation.
Observe the Floc
Operators should observe:
- when floc begins forming;
- floc size;
- floc strength;
- whether floc breaks apart;
- whether floc settles effectively.
Clarification Control
Clarifiers should remove a large portion of properly formed floc before filtration.
Monitor:
- clarified-water turbidity;
- surface conditions;
- flow distribution;
- sludge accumulation;
- sludge withdrawal;
- outlet-weir condition.
High Clarified-Water Turbidity
Possible causes include:
- poor coagulation;
- poor flocculation;
- high hydraulic loading;
- short-circuiting;
- sludge carryover;
- mechanical failure.
Do Not Correct a Mechanical Problem with More Chemical
Increasing coagulant dose will not repair:
- a failed sludge collector;
- a blocked sludge line;
- uneven outlet weirs;
- severe hydraulic short-circuiting.
Filtration Control
Key filter indicators include:
- individual filter turbidity;
- filtration rate;
- head loss;
- filter-run length;
- post-backwash turbidity.
Filter Problems May Begin Upstream
If several filters develop problems at the same time, review common upstream conditions before assuming several filters failed simultaneously.
One Filter Versus All Filters
If only one filter has high turbidity, investigate that filter for:
- media problems;
- underdrain problems;
- valve problems;
- uneven backwashing;
- instrument problems.
If all filters deteriorate together, investigate:
- coagulation;
- flocculation;
- clarification;
- raw-water changes.
Filter Head Loss
Head loss normally increases as solids accumulate.
Rapid head-loss development can indicate:
- high solids loading;
- poor clarification;
- poor coagulation;
- media fouling.
Short Filter Runs
Shorter-than-normal filter runs can indicate:
- increased particle loading;
- poor pretreatment;
- higher filtration rate;
- media problems;
- ineffective backwashing.
Post-Backwash Problems
If turbidity remains high after backwash, review:
- backwash effectiveness;
- media disturbance;
- filter-to-waste operation where applicable;
- upstream treatment;
- turbidimeter condition.
Disinfection Control
Disinfection performance depends on more than chemical feed alone.
Important factors can include:
- disinfectant dose;
- disinfectant demand;
- residual;
- contact time;
- pH;
- temperature;
- upstream particle removal.
Chlorine Demand
A simplified relationship is:
Chlorine Demand = Chlorine Dose - Chlorine Residual
Residual Falls Unexpectedly
Possible causes include:
- higher chlorine demand;
- lower chemical feed;
- feed-pump problem;
- chemical-strength change;
- sample or instrument problem.
Upstream Treatment Supports Disinfection
Good particle removal helps reduce the treatment burden on disinfection.
Operators should not rely on increased disinfectant dose as a substitute for poor coagulation, clarification, or filtration.
Use Process Trends
Trend data can reveal relationships that are difficult to see from single measurements.
Useful trends include:
- raw-water turbidity versus coagulant dose;
- coagulant dose versus clarified-water turbidity;
- clarified-water turbidity versus filter-run length;
- filter head loss versus time;
- chlorine dose versus residual;
- plant flow versus treatment performance.
Establish Normal Operating Patterns
Troubleshooting is easier when operators know what normal operation looks like.
Normal patterns can include:
- typical chemical doses;
- typical filter-run length;
- normal head-loss development;
- normal clarified-water turbidity;
- normal disinfectant demand.
Do Not Use Normal Values as Fixed Rules
Normal operating ranges can change as raw-water conditions change.
The objective is to understand process behavior, not to hold every operating value constant regardless of water quality.
Alarm Response
An alarm should trigger evaluation, not automatic assumptions.
First determine:
- what parameter alarmed;
- whether the reading is credible;
- whether related parameters changed;
- whether equipment status changed.
Verify Unexpected Instrument Readings
Check:
- sample flow;
- sensor cleanliness;
- calibration;
- electrical or communication status;
- manual or laboratory confirmation where appropriate.
Instrument Failure Versus Process Failure
One abnormal sensor with otherwise stable process conditions can suggest an instrument problem.
Several related measurements changing together can provide stronger evidence of a real process upset.
Avoid Overcorrection
Large or rapid treatment changes can make an unstable process even more difficult to control.
When conditions permit, make controlled adjustments and observe the response.
Change One Major Variable at a Time
If several variables are changed simultaneously, it becomes difficult to determine which change improved or worsened treatment.
Document Operator Adjustments
Record:
- what changed;
- when it changed;
- why it changed;
- the new setting;
- the process response.
Example: Raw-Water Turbidity Rises Rapidly
Review:
- plant flow;
- coagulant dose;
- pH and alkalinity;
- jar-test results;
- floc formation;
- clarifier loading;
- filter performance.
Example: Clarified-Water Turbidity Rises but Raw Water Is Stable
Investigate:
- chemical feed;
- rapid mixing;
- flocculation;
- clarifier hydraulics;
- sludge removal;
- mechanical equipment.
Example: All Filters Develop Short Runs
A common upstream cause is likely.
Review:
- coagulation;
- clarification;
- raw-water solids;
- plant flow.
Example: One Filter Has High Turbidity
Review that filter's:
- turbidity instrument;
- flow;
- media;
- backwash history;
- underdrains;
- valves.
Example: Chlorine Residual Drops Across the Plant
Review:
- chlorine feed;
- chemical supply;
- raw-water or process demand;
- flow;
- analyzer condition.
Example: pH Drops After Coagulant Dose Is Increased
The added coagulant may be consuming additional alkalinity.
Review:
- raw-water alkalinity;
- coagulant dose;
- pH;
- jar-test results.
Example: Treatment Deteriorates During Cold Weather
Cold water can affect:
- coagulation reactions;
- floc formation;
- settling;
- backwash expansion.
Review several processes rather than adjusting only one parameter.
Example: Filter Head Loss Rises Quickly After a Storm
Possible causes include:
- higher raw-water solids;
- clarifier carryover;
- poor coagulation;
- higher filter loading.
Example: Chemical Use Increases but Treatment Does Not Improve
Verify:
- chemical feed calibration;
- chemical concentration;
- pH;
- mixing;
- application point;
- whether the actual problem is mechanical or hydraulic.
Example: Instrument Reading Changes Instantly
A sudden change without corresponding process evidence should prompt instrument verification.
Do not ignore the reading, but do not make a major treatment change without evaluating whether it is credible.
Process Control During Flow Changes
When plant flow changes, operators should review:
- chemical-feed pacing;
- clarifier loading;
- filter loading;
- contact time;
- pump and valve operation.
Flow-Paced Chemical Feed
Flow pacing can help maintain a target dose as plant flow changes.
However, a perfectly flow-paced pump can still deliver the wrong treatment if the target dose itself is no longer appropriate for the water quality.
Automatic Control Systems
Automation can help control:
- flow;
- chemical feed;
- filter rates;
- valves;
- alarms.
Operators remain responsible for evaluating whether automatic responses make sense under actual process conditions.
Do Not Trust Automation Blindly
A bad sensor signal can cause an automatic control system to make a bad adjustment.
Operators should understand:
- what the control loop measures;
- what equipment it controls;
- how to recognize abnormal behavior.
Process Control and Maintenance
Treatment problems can be caused by equipment failure.
Examples include:
- failed chemical-feed pumps;
- poor mixers;
- clarifier collector failure;
- filter valve problems;
- failed turbidimeters;
- disinfectant analyzer problems.
Good Maintenance Supports Stable Treatment
Preventive maintenance helps reduce process upsets caused by mechanical or instrumentation failure.
Shift Handoffs
Operators should communicate:
- raw-water changes;
- chemical adjustments;
- abnormal turbidity;
- filters out of service;
- alarms;
- equipment problems;
- expected weather conditions.
Operating Records
Useful records include:
- raw-water data;
- plant flow;
- chemical doses;
- jar-test results;
- clarifier turbidity;
- filter turbidity;
- filter head loss;
- disinfectant dose and residual;
- operator adjustments.
Troubleshoot from Upstream to Downstream
A practical sequence is:
- verify the abnormal result;
- review raw-water conditions;
- review plant flow;
- review chemical feed;
- review coagulation and flocculation;
- review clarification;
- review filtration;
- review disinfection;
- check instrumentation and equipment;
- make controlled corrections and monitor response.
Common Surface-Water Treatment Control Mistakes
- Looking only at the process where the problem becomes visible.
- Ignoring raw-water changes.
- Changing chemical dose without checking flow.
- Changing several variables at once.
- Using more chemical to compensate for a mechanical problem.
- Ignoring individual filter data.
- Making large adjustments from one questionable instrument reading.
- Relying on automation without understanding the control logic.
- Failing to document operator changes.
- Waiting for finished-water deterioration before responding to upstream warning signs.
A Practical Process-Control Review
- Review raw-water quality.
- Review plant flow.
- Verify chemical-feed rates.
- Observe coagulation and flocculation.
- Review clarified-water turbidity.
- Review individual filter performance.
- Review disinfectant residual.
- Check alarms and equipment status.
- Compare current results with normal trends.
A Practical Treatment-Upset Review
- Verify the abnormal measurement.
- Identify when the problem began.
- Determine whether raw water changed.
- Determine whether plant flow changed.
- Review recent chemical adjustments.
- Review upstream treatment processes.
- Check equipment and instrumentation.
- Make one controlled correction where practical.
- Measure the response.
- Document the result.
A Practical High-Turbidity Review
- Identify where turbidity first becomes abnormal.
- Check raw-water turbidity.
- Check coagulation and pH.
- Observe flocculation.
- Review clarifier performance.
- Determine whether one or all filters are affected.
- Check filter flow, head loss, and backwash history.
- Verify turbidity instruments.
A Practical Chemical-Feed Review
- Verify plant flow.
- Verify the target dose.
- Calculate the required chemical feed.
- Verify solution strength.
- Check feed-pump calibration.
- Check the injection point.
- Review the process response.
What to Remember for the Exam
- Surface-water treatment requires continuous process control because source-water quality can change rapidly.
- Treatment processes should be evaluated as an integrated treatment train.
- Troubleshooting should usually begin with raw-water conditions and proceed downstream.
- Plant flow affects chemical feed, clarifier loading, filter loading, and detention time.
- Chemical feed in lb/day can be calculated as MGD × mg/L × 8.34.
- Coagulation performance depends on dose, pH, alkalinity, raw-water quality, and mixing.
- Jar testing helps evaluate treatment changes before full-scale adjustment.
- Clarified-water turbidity can provide early warning of treatment deterioration.
- High solids loading from clarification can shorten filter runs.
- One poorly performing filter suggests a local filter problem, while all filters changing together suggests a common upstream cause.
- Filter head loss normally increases as solids accumulate.
- Disinfection performance depends on dose, demand, residual, contact time, and upstream particle removal.
- Chlorine demand can be expressed as dose minus residual.
- Operators should use trends rather than isolated measurements whenever possible.
- Unexpected instrument readings should be verified before major process changes.
- Automatic controls depend on accurate sensors and should not be trusted blindly.
- Mechanical failures can create water-quality problems.
- Large or simultaneous process adjustments can make troubleshooting more difficult.
- Operator adjustments should be documented so their effects can be evaluated.
- Good surface-water treatment control combines raw-water monitoring, chemical feed, clarification, filtration, disinfection, instrumentation, maintenance, trend analysis, and disciplined troubleshooting.