The short answer
Residuals
Residuals are the solids a plant removes, primary sludge and waste activated sludge, and the process that turns them into something that can be hauled, thickened to reduce volume, digested to stabilize and reduce them, and dewatered to a cake. The controls run each step as a sequence with its loops and interlocks: a thickener on a feed schedule with a polymer dose paced to the solids loading and an underflow pumped on a blanket or a density target; a digester held at temperature with heat exchangers and mixed on a schedule, its gas collected, measured, and burned or flared with the safety systems of a fuel gas plant; a dewatering unit started in a sequence of conveyor, polymer, feed, and press with the polymer dose tuned to the cake solids and the centrate quality; and storage and hauling with levels, weights, and the records the biosolids regulations require. The measurements that make it controllable are solids concentration and flow, from which every loading is computed, and the trends of cake solids, polymer per dry ton, and digester volatile solids reduction are what the operators manage against.
Key points
- Solids loading, flow times concentration, is the number every residuals loop is paced to; measure both.
- Thickening, digestion, and dewatering are each a sequence with interlocks; the polymer dose is paced to dry solids and trimmed on performance.
- Digesters are fuel gas plants: gas detection, pressure relief, flame arrestors, flares, and classified electrical equipment, all interlocked.
- Dewatering performance is judged on cake solids, capture, and polymer per dry ton, trended from the instruments and the laboratory.
- Biosolids records, quantities, pathogen and vector attraction reduction, and destinations, come from the historian and the hauling log.
The train
| Step | Equipment | What the controls run |
|---|---|---|
| Thickening | Gravity thickener, dissolved air flotation, rotary drum, gravity belt | Feed pumps and schedule, polymer dose to loading, underflow or float removal on blanket or density, overflow quality |
| Aerobic digestion | Aerated tanks, often at small plants | Aeration for dissolved oxygen and mixing, decant sequence, temperature, volatile solids record |
| Anaerobic digestion | Heated, mixed tanks with gas handling | Feed schedule, temperature loop, mixing, gas pressure, gas flow, flare and boiler, safety systems |
| Dewatering | Belt filter press, centrifuge, screw press | Start and stop sequence, feed and polymer pacing, machine variables such as belt speed or bowl differential, cake conveyor, washwater |
| Storage and hauling | Cake hoppers, storage pads, liquid storage | Levels and weights, loadout sequence, truck records |
| Sidestream | Centrate and filtrate return | Flow and load back to the plant; equalization to avoid an ammonia slug |
Loading
Dry solids (lb/day) = Q (gpd) × TSS (mg/L) × 8.34 ÷ 1,000,000
- Q = sludge flow in gallons per day
- TSS = total suspended solids in milligrams per liter, from a solids probe verified against the laboratory
- Polymer is dosed in pounds of active polymer per dry ton of solids; the controller computes it
Every residuals loop is paced to dry solids: the polymer pump follows the product of flow and concentration, the digester feed schedule is set in pounds per day of volatile solids, and the dewatering feed rate is set in dry solids per hour. A solids probe on the feed line, verified weekly against the laboratory, and a flowmeter on the same line are the instruments that make this possible; without them the dose follows the flow alone and the polymer use swings with every change in concentration.
Digesters
An anaerobic digester is held at its temperature, mesophilic around 35 degrees Celsius in most plants, by circulating sludge through a heat exchanger with a boiler or a heat recovery source, controlled on the digester temperature with the exchanger outlet limited to avoid baking the sludge. It is mixed by gas, pumps, or mechanical mixers on a schedule or continuously; fed on a schedule that spreads the daily load; and its gas is collected at a low pressure held by the gas system, measured, and used in a boiler or an engine with the excess flared. The controls interlock the whole gas system: pressure relief and vacuum relief on the cover, flame arrestors, a flare with its own pilot and flame detection, gas detection in the building with ventilation interlocks, and classified electrical equipment. A digester upset shows first in the gas: falling production, rising carbon dioxide fraction, and a falling pH, and the trend of those with the volatile acids from the laboratory is the alarm.
Dewatering
- Sequence
- Conveyor and cake handling first, then washwater and belts or bowl, then polymer, then feed; on stop, feed off, polymer off, a flush, then the machine and the conveyor. Every step with feedback and a timeout.
- Polymer
- Made down from neat polymer or emulsion with a make-down system that has its own sequence, aged, and dosed in pounds per dry ton, paced to the feed solids loading and trimmed by the operator on cake solids and centrate quality.
- Machine variables
- Belt speed and tension, or bowl speed and differential, or screw speed and back pressure; set by the operator within limits, with torque, pressure, and vibration alarms and trips.
- Performance
- Cake solids from the laboratory, capture from feed and centrate solids, polymer per dry ton from the totals, and machine hours; trended so that a drifting performance is seen before the hauling costs rise.
Safety systems
- Gas detection for methane and hydrogen sulfide in digester buildings and galleries, with ventilation and equipment shutdown interlocks and alarms to the callout.
- Digester cover pressure and vacuum relief, level and cover position monitoring, and a foam detection where foaming is a risk.
- Flare flame detection, pilot supervision, and a purge sequence.
- Emergency stops on conveyors and presses; guards interlocked.
- Confined space and lockout considerations built into the sequence permissives where equipment can be entered.
Records
Biosolids regulations require records of the quantities produced and hauled, the treatment achieved for pathogen and vector attraction reduction, the sampling results, and the destinations. The historian holds the digester temperature and detention, the volatile solids reduction inputs, the dewatering totals, and the hauling weights where a scale is integrated, and the report is built from them with the laboratory results. A digester temperature record with a gap on the day the pathogen reduction is claimed is a compliance problem, so the residuals tags get the same collection care as the effluent tags.
Frequently asked questions
- Can polymer dosing be closed-loop?
- Partly: the dose is paced to dry solids automatically, and some plants trim it on centrate turbidity or a charge analyzer. Cake solids from the laboratory remain the final judge, and the operator adjusts the pounds per dry ton against them.
- Why is the digester gas production falling?
- Less feed, a temperature drop, a toxic load, or an acid buildup; the temperature and feed trends and the laboratory volatile acids and alkalinity say which. Falling gas with rising carbon dioxide and falling pH is an upset that needs feed reduction and possibly alkalinity addition.
- How do I know the thickener is working?
- Underflow concentration from the solids probe against the target, overflow suspended solids, and the blanket level; polymer per dry ton for a conditioned thickener. A rising overflow solids and a falling underflow concentration is a loaded or upset thickener.
- What should be on the generator in the residuals area?
- The digester gas safety systems, the ventilation, the flare, the heating circulation where a temperature drop would matter, and the gas detection. Dewatering can usually wait; the digester cannot be left without its safety systems.
Related topics
- RAS and WAS ControlReturn and waste activated sludge: what each flow does for the process, RAS control by influent flow ratio or by blanket level, WAS control to a target sludge age or a target mixed liquor concentration, the flow and solids measurements each needs, and the pump and valve arrangements that make the setpoints achievable.
- Chemical Systems in Wastewater TreatmentThe chemical feeds of a wastewater plant and their controls: coagulants for phosphorus, polymer for thickening and dewatering, hypochlorite and sulfite for disinfection and dechlorination, and the alarms that keep the plant in its permit.
- Clarifier ControlPrimary and secondary clarifiers from the control room: the mechanism drive and its torque protection, sludge blanket measurement and sludge withdrawal control, scum removal, the weir and the effluent turbidity, and the sludge blanket as the early warning for a secondary process losing its footing.
- Sequential Function ChartThe IEC 61131-3 language for sequences: steps, transitions, actions and their qualifiers, alternative and simultaneous branches, where it fits in water and wastewater, and the rules that keep a chart from sticking in a step.
- UL 698AIndustrial control panels for hazardous locations: what UL 698A adds to UL 508A, a panel in a classified area or a safe-area panel with intrinsically safe circuits, barriers and isolators, the control drawing, and where utilities meet classified areas.
- EPA Rules and Their Control System ImplicationsThe federal environmental agency rules that shape what a water or wastewater control system must measure, record, and report: the surface water rules, residual and byproduct rules, groundwater and lead rules, discharge permits, and record retention.
Direct contact
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