Managing biosolids safely when wet weather arrives
Rain can change a wastewater facility’s operating conditions within hours. Influent flows rise, collection systems surcharge, treatment capacity narrows, and solids that normally move through a predictable process may become difficult to thicken, dewater, store, or transport. For agencies in the Los Angeles Basin, short intense storms can create especially demanding conditions because dense development leaves limited room for hydraulic recovery.
Biosolids management during wet weather events depends on preparation before the first major inflow. Facilities need clear decision thresholds, reliable communication, available storage, and an operating strategy that protects treatment performance without creating an unsafe backlog of solids.
The strongest programs connect collection system information with plant operations, laboratory data, hauling logistics, and end-use requirements. They also treat wet weather as a cross-functional event involving operators, engineers, maintenance teams, laboratory staff, contractors, and agency leadership.
Understand the wet weather risk
A useful plan begins with a site-specific risk assessment. Review historical rainfall, influent flow records, peak solids production, storage volumes, dewatering capacity, truck access, and the time required to secure additional hauling or disposal options. Identify which tanks, channels, pumps, digesters, and loading areas become vulnerable during high-flow conditions.
Rain can affect biosolids indirectly as well as directly. Infiltration and inflow may dilute incoming solids, alter polymer demand, and reduce the consistency of feed to thickening or dewatering equipment. Stormwater entering solids handling areas can add free water to stockpiles and increase contaminated runoff. Flooding may also restrict access to drying beds, cake storage, or truck loading facilities.
Use defined trigger levels rather than relying on informal judgment. Examples include a forecast rainfall threshold, a rising wet well level, a percentage of available storage, a decline in cake solids, or a projected delay in hauling. Each trigger should identify who makes the operating decision, who receives the notification, and what action follows.
Protect hydraulic and process capacity
Wet weather operations should preserve the treatment train’s ability to receive flow while avoiding unnecessary movement of dilute solids. Operators can review return activated sludge rates, waste activated sludge schedules, digester loading, thickener performance, and dewatering priorities as conditions change. Any adjustment should be based on process data and documented so that the next shift understands the reason for the change.
A clear view of process connections is essential when bypassing, rerouting, or temporarily changing equipment priorities. Teams can use process flow diagrams to verify valves, pumps, overflow paths, isolation points, and the consequences of taking a unit out of service. This is particularly valuable when storm conditions create pressure to act quickly.
Avoid treating every increase in flow as a reason to maximize solids wasting. Excessive wasting may overload downstream equipment, while delaying wasting too long can affect biological treatment and settleability. Operators should compare solids inventory, oxygen demand, settling results, equipment capacity, and expected storm duration before changing schedules.
Preventive maintenance also supports wet weather resilience. Pumps, level sensors, polymer systems, centrifuges, belt presses, conveyors, and odor control equipment should be inspected before the wet season. Backup power, fuel, lighting, washdown water, and spill response supplies deserve the same attention as primary treatment assets.
Control storage, dewatering, and movement
Storage is often the decisive factor in biosolids response. Calculate usable capacity rather than relying on nominal tank volume. Account for mixing requirements, dead zones, freeboard, access restrictions, incompatible materials, and the time needed to empty a unit. Maintain a current inventory that shows both volume and estimated dry tons, since wet-weather dilution can make volume alone misleading.
Wet weather may require a deliberate shift in dewatering priorities. A facility might process the oldest or most stable material first, reserve a unit for emergency holding, or operate equipment at a lower loading rate to maintain cake quality. Changes should consider polymer consumption, filtrate or centrate return loads, odor potential, and the ability of downstream treatment to absorb recycle flows.
Storage tanks and basins should be evaluated for actual mixing and usable capacity, not just design assumptions. Facilities can apply tracer studies to identify short-circuiting, stagnant zones, and effective detention time. The findings can support better level setpoints, mixing practices, and estimates of how long a tank can safely accommodate additional material.
Keep solids moving through a controlled chain of custody. Confirm container labeling, sampling requirements, load documentation, approved destinations, and truck availability before storage reaches a critical level. If land application, composting, landfill disposal, or another beneficial-use pathway is temporarily unavailable, the contingency route should already be reviewed for capacity and compliance.
| Operating area | Wet-weather concern | Practical control | Useful indicator |
|---|---|---|---|
| Thickening | Dilute feed and unstable solids capture | Adjust feed rate and polymer based on testing | Capture rate and sludge concentration |
| Dewatering | Lower cake solids and higher recycle load | Prioritize equipment reliability and verify polymer dose | Cake solids and centrate quality |
| Storage | Rapid loss of free capacity | Track wet volume, dry tons, freeboard, and mixing | Hours of available capacity |
| Hauling | Delayed access or contractor constraints | Prearrange backup routes and loading windows | Confirmed trucks and destination capacity |
| Odor control | Longer residence time and anaerobic conditions | Increase inspection, mixing, and response readiness | Complaints, sulfide, and visual conditions |
| Documentation | Fast decisions made without a record | Use shift logs and event checklists | Complete, time-stamped records |
Coordinate reuse and contingency outlets
End-use planning should account for weather beyond the facility fence line. Rain can make agricultural fields inaccessible, interrupt composting operations, delay landfill acceptance, or affect the quality requirements of a receiving site. A biosolids program needs multiple approved outlets and current contact information for each provider, including after-hours escalation procedures.
Water reuse and biosolids operations are often discussed separately, yet both influence a facility’s resilience and resource recovery goals. Understanding the value of reclaimed water can help agencies evaluate broader operational priorities, especially where industrial reuse, cooling demand, and treatment capacity intersect. Better coordination of water and solids programs can strengthen the business case for reliability investments.
Before a storm, verify destination conditions rather than assuming normal service will continue. Confirm receiving hours, road access, weather restrictions, documentation, sampling windows, and acceptable material characteristics. If a load is rejected, staff should know where it can be held safely and which alternate destination can accept it.
Communication with regulators, neighboring agencies, contractors, and the public should be timely and factual. Report unusual releases, spills, odors, or changes in approved management pathways according to applicable requirements. A concise incident record can demonstrate control of the event and support later review.
Build a practical response routine
A written procedure is useful only when staff can apply it under pressure. Conduct tabletop exercises before the wet season, using realistic scenarios such as a forecast atmospheric river, a failed centrifuge, a flooded access road, or a hauling contractor cancellation. Include operations, maintenance, laboratory, safety, procurement, communications, and management personnel.
Training should cover safe work around tanks, slippery surfaces, mobile equipment, confined spaces, biological hazards, and high-pressure washdown. Operators should understand when to stop work, how to report changing conditions, and who has authority to redirect material. Lessons from exercises and previous storms should be incorporated into the next revision of the plan.
Actions that strengthen readiness
- Set rainfall, flow, storage, and solids-inventory trigger points before the event.
- Verify backup power, pumps, mixers, dewatering equipment, instruments, and spill supplies.
- Maintain current agreements with haulers, disposal facilities, composters, and beneficial-use partners.
- Use shift checklists that record levels, equipment status, cake quality, odors, and decisions.
- Schedule a post-event review covering performance, safety, costs, communications, and corrective actions.
A strong response also depends on good data. Record influent flow, solids concentrations, polymer use, cake production, storage levels, hauling activity, equipment failures, and unusual odors at useful intervals. Comparing these records with rainfall and forecast data can reveal where the facility needs more capacity, better controls, or different staffing.
Put readiness into practice
Wet weather biosolids management is a continuing operational discipline, not a single emergency procedure. Facilities that regularly inspect assets, test assumptions, train personnel, and maintain relationships with receiving sites are better positioned to protect water quality and keep solids moving when conditions deteriorate.
LABS of CWEA provides a valuable setting for water and wastewater professionals to exchange practical experience through technical presentations, facility tours, workshops, automation training, and professional development opportunities. Agencies can use these connections to refine storm procedures, strengthen operator knowledge, and share solutions that fit the realities of Southern California facilities.
Review your wet-weather plan before the next forecasted storm, walk the solids handling route with the response team, and convert the highest-risk gaps into assigned actions with deadlines. A prepared facility gives operators clearer choices, protects the public and environment, and turns a difficult weather event into a manageable operating challenge.