2.11 Water Treatment Plant Residuals: Lagoons, Drying Beds, Land Application & Recycle

Every gallon treated produces a residual

A drinking water plant does not destroy contaminants; it concentrates them. Understanding what leaves the plant in the solids stream, and where it goes, is a listed WPI job task and a genuine compliance exposure — a plant that quietly dumps basin sludge to a storm drain has created an unpermitted discharge.

Residual streams from a conventional plant

Alum sludge is the hardest of these to handle. Aluminum hydroxide floc is a gelatinous, highly hydrated solid that holds water tenaciously. It typically leaves a sedimentation basin at 0.5 to 2 percent solids, which is 98 to 99.5 percent water, and it resists mechanical dewatering. Ferric sludge behaves similarly but is somewhat easier. Lime softening sludge, by contrast, is crystalline calcium carbonate that thickens and dewaters readily, sometimes reaching 50 percent solids or more on a drying bed, and it can often be reused — as agricultural liming agent, as feedstock for lime recalcining at large plants, or as a soil amendment.

Sludge quantity is estimated with the pounds formula applied to what was removed and what was added:

lbs/day of solids ≈ (flow, MGD) × (turbidity or suspended solids removed + coagulant contribution, mg/L) × 8.34

A common exam approximation credits roughly 0.44 pounds of dry solids per pound of alum fed, plus the raw water suspended solids removed.

Handling and treatment

Equalization comes first: backwash and basin blowdown arrive as short, high-rate slugs, and a washwater recovery or equalization basin converts them to a steady flow the rest of the residuals system can handle.

Gravity thickening in a circular thickener with a slow-turning rake concentrates alum sludge from around 1 percent to perhaps 2 to 4 percent solids and produces a clear supernatant. Polymer conditioning improves both the settled solids concentration and supernatant clarity.

Drying beds are sand or paved beds where sludge is spread in layers and dewatered by drainage plus evaporation. They are cheap, need no mechanical equipment, and work particularly well in Colorado's high, dry, sunny climate. Their limitations are land area and weather dependence. One useful Colorado phenomenon: freeze-thaw conditioning. When alum sludge freezes, the ice crystals rupture the gelatinous floc structure irreversibly, and on thawing the sludge releases water readily and can reach 20 percent solids or more. Beds that operate through a Colorado winter often dewater far better than the same beds in a mild climate.

Lagoons are larger earthen basins operated on fill, settle, decant, and dry cycles. They provide storage as well as dewatering, and cleanout may be on a multi-year cycle.

Mechanical dewatering — belt filter presses, centrifuges, plate-and-frame presses, and screw presses — is used where land is unavailable. Plate-and-frame presses achieve the highest cake solids on alum sludge, often 30 to 40 percent, but they are batch machines with high labor and polymer demand.

The Filter Backwash Recycle Rule

Recycling residual liquids is attractive — it recovers water, which matters in Colorado — but it also returns concentrated pathogens and solids to the head of the plant. The federal Filter Backwash Recycle Rule (FBRR) governs the practice for systems using conventional or direct filtration, and Colorado implements it through Regulation 11.

The core requirement: spent filter backwash water, thickener supernatant, and liquids from dewatering processes must be returned to a location upstream of the primary coagulant addition point, unless the state specifically approves an alternative location. The reason is simple and is the whole point of the rule: recycled flow must pass through all the plant's treatment processes, especially coagulation and filtration, so that any Cryptosporidium oocysts concentrated in the residuals are removed rather than short-circuited into the clearwell.

Systems must also keep records of the recycle location, flow rate, and duration, and be prepared to demonstrate that the recycle does not degrade finished water quality. Operationally, the recycle stream is fed back at a controlled, steady rate — generally not more than about 10 percent of raw water flow — because slugging a plant with concentrated backwash upsets coagulation, spikes turbidity, and can drive filter breakthrough.

Disposal pathways

1. Discharge to a receiving stream. This is a point-source discharge and requires a CDPS permit under Regulation 61, with limits typically on total suspended solids, pH, and sometimes aluminum, iron, and manganese. Untreated basin blowdown almost never meets those limits.
2. Discharge to the sanitary sewer. Common and often the simplest route, but requires the receiving wastewater utility's agreement. Alum sludge can upset a wastewater plant's solids handling, and ion exchange brine can jeopardize the wastewater plant's own chloride and total dissolved solids limits.
3. Landfill. Dewatered cake goes to a permitted solid waste facility. The landfill will require a paint filter liquids test pass and characterization; residuals bearing radium or uranium from groundwater treatment may be classified as technologically enhanced naturally occurring radioactive material (TENORM) and require special handling.
4. Land application. Alum and lime residuals can be applied to agricultural land as a soil amendment. Lime residual is genuinely valuable for raising soil pH. Alum residual can bind soil phosphorus, which is beneficial where phosphorus runoff is a concern and detrimental where crops need available phosphorus. Site approval, agronomic rates, buffer distances, and metals characterization apply.
5. Beneficial reuse. Lime recalcining recovers quicklime at very large softening plants; calcium carbonate residual is sold as agricultural lime; some alum residuals are used in topsoil blends and turf establishment.
6. On-site disposal and composting. Monofills on plant property and co-composting with organics are both listed in the WPI outline as options an operator may be managing.

What the operator actually monitors

• Basin blowdown frequency and duration, set so the sludge blanket never rises into the settled water and never becomes so deep that it goes septic and releases manganese and taste and odor compounds back into the water.
• Thickener underflow solids and supernatant turbidity, which tell you whether polymer dose and rake speed are right.
• Drying bed loading depth and turnover schedule.
• Recycle flow rate as a percentage of raw water flow, and the resulting effect on settled water turbidity.
• Cake solids and polymer dose where mechanical dewatering is used.
• Records — quantities produced, hauled, and applied, and the analytical characterization supporting each disposal route.