2.9 Water Treatment Residuals: Handling, Thickening & Disposal
Key Takeaways
- Alum sludge production can be estimated as pounds per day equal to flow in MGD times 8.34 times the sum of turbidity-derived solids, roughly 0.44 pounds of solids per pound of alum dosed, plus any added polymer or powdered carbon.
- The Filter Backwash Recycling Rule requires recycled backwash water, thickener supernatant, and liquids from dewatering to be returned to a point ahead of the primary coagulant addition unless the state approves an alternate location.
- Alum sludge dewaters poorly because aluminum hydroxide floc is gelatinous and binds water, so freeze-thaw conditioning, polymer conditioning, or lime addition is normally required before mechanical dewatering.
- Lime softening sludge is dense, settles and dewaters far better than alum sludge, and can be recalcined at large plants to recover quicklime and carbon dioxide.
- Membrane concentrate and spent ion-exchange brine are high-TDS liquid residuals whose disposal is generally limited to sanitary sewer discharge, permitted surface water discharge, deep well injection, or evaporation ponds.
Water Treatment Residuals: Handling, Thickening & Disposal
Every pound of turbidity and every pound of coagulant that enters a plant has to leave as residuals. The ABC Water Treatment outline names "plant residuals," "residuals disposal (e.g., lagoons, sludge drying beds, land application, on-site disposal, solids composting)," and "monitor and control residual effluents to comply with regulatory permit limits" - the last of which sits in the Security, Safety, and Administrative Procedures area because it is a permit obligation.
1. What a Water Plant Produces
| Residual stream | Typical volume | Character |
|---|---|---|
| Sedimentation basin sludge | 0.3 to 1 percent of plant flow | 0.1 to 2 percent solids; gelatinous if alum, dense if lime |
| Filter backwash water | 2 to 5 percent of plant flow | Very dilute, 30 to 300 mg/L solids |
| Filter-to-waste | 0.2 to 1 percent | Nearly clean, slightly turbid |
| Spent regeneration brine (ion exchange) | 1 to 5 percent | Very high TDS, high hardness or nitrate |
| Membrane concentrate | 3 to 30 percent | High TDS, concentrated by the recovery ratio |
| Spent GAC / media | Batch | Solid; may be reactivated or landfilled |
2. Estimating Coagulation Sludge Quantity
The standard estimating expression treats sludge as the sum of the solids removed from the raw water plus the solids created by the coagulant.
Sludge (lb/day) = Flow (MGD) x 8.34 x [ (K x Raw turbidity, NTU) + (0.44 x Alum dose, mg/L) + Polymer dose + PAC dose ]
- K converts turbidity to suspended solids and is site-specific, typically 0.5 to 2.0 mg/L TSS per NTU; determine it by measuring both on the same samples.
- 0.44 is the pounds of aluminum hydroxide solids produced per pound of commercial alum dosed. For ferric chloride the factor is about 0.54, and for ferric sulfate about 0.5.
Worked example
A 6.0 MGD plant treats raw water at 30 NTU (site K = 1.0), doses 35 mg/L alum and 0.5 mg/L polymer.
- Turbidity solids = 1.0 x 30 = 30 mg/L
- Alum solids = 0.44 x 35 = 15.4 mg/L
- Polymer = 0.5 mg/L
- Total = 45.9 mg/L
- Sludge = 6.0 x 8.34 x 45.9 = 2,297 lb/day (dry solids)
At 1.0 percent solids, that is 2,297 / (0.01 x 8.34) = 27,540 gallons per day of wet sludge - which is why thickening matters.
3. Backwash Water Recycle and the Filter Backwash Recycling Rule
Most plants recover backwash water rather than discharge it, because 2 to 5 percent of production is a lot of water to throw away. The Filter Backwash Recycling Rule (FBRR) governs how.
Recycled filter backwash water, thickener supernatant, and liquids from dewatering processes must be returned to a location such that all processes of a system's conventional or direct filtration - including coagulation, flocculation, sedimentation, and filtration - are employed. In practice that means the return point must be ahead of the primary coagulant addition, unless the state specifically approves an alternate location.
The reason is pathogen concentration: backwash water contains the Cryptosporidium oocysts the filters just removed. Returning it downstream of coagulation reinjects them past the barrier.
Operational rules for recycle:
- Equalize the return. Dumping a 5-minute backwash volume back into a 4 MGD plant creates a hydraulic and solids spike. Recycle at a controlled, low, continuous rate - commonly capped at 10 percent or less of plant influent flow.
- Settle first. A washwater recovery basin lets solids settle so only the supernatant is recycled and the underflow goes to the residuals train.
- Watch the loop. Recycling concentrates manganese, TOC, and DBP precursors. If recycle rate rises and finished manganese starts creeping, the loop is the first suspect.
4. Thickening and Dewatering
The character problem
Alum sludge is the hardest common sludge to dewater. Aluminum hydroxide floc is amorphous and gelatinous, holds bound water, and compresses under pressure rather than releasing water. Untreated alum sludge thickens from perhaps 0.5 percent to only 2 to 4 percent solids by gravity, and mechanical dewatering rarely exceeds 15 to 25 percent cake without conditioning.
Lime softening sludge is the opposite - calcium carbonate crystals are dense (specific gravity around 2.7), settle quickly, thicken to 10 to 30 percent by gravity alone, and dewater readily.
Thickening and dewatering options
| Method | Typical output solids | Notes |
|---|---|---|
| Gravity thickener | 2 to 4% (alum), 10 to 30% (lime) | Continuous, low energy; picket-fence rake releases entrained water |
| Lagoon | 5 to 15% after long-term consolidation | Cheapest; needs land and years of storage; decant supernatant back to plant |
| Sand drying bed | 20 to 40% after weeks | Evaporation plus underdrainage; weather-dependent |
| Freeze-thaw bed | 20 to 30% | Freezing irreversibly restructures alum floc, releasing bound water - highly effective in Virginia's mountain counties, unreliable in Tidewater |
| Belt filter press | 12 to 20% (alum), 50 to 65% (lime) | Continuous, needs polymer conditioning |
| Centrifuge | 15 to 25% (alum) | Compact, high energy, high polymer demand |
| Plate-and-frame (pressure) filter press | 30 to 45% (alum) | Batch, highest cake solids, highest capital and labor |
Conditioning
- Polymer - cationic polymers at 5 to 20 lb per dry ton for belt presses and centrifuges.
- Lime - raises pH, adds inert dense solids, improves cake release and reduces odor.
- Freeze-thaw - the only conditioning that permanently changes alum floc structure.
5. Lime Softening Sludge and Recalcination
Lime softening generates by far the largest sludge mass of any water treatment process, because the calcium removed from the water precipitates alongside the lime added to remove it. Roughly 2 pounds of CaCO3 sludge is produced per pound of hardness removed as CaCO3, plus magnesium hydroxide if magnesium is removed.
Recalcination heats dewatered CaCO3 sludge in a furnace at about 1,800 degrees F:
CaCO3 + heat -> CaO + CO2
The quicklime is reused for softening and the carbon dioxide is captured and reused for recarbonation. Recalcination is only economic at very large plants, and only when the sludge is predominantly calcium carbonate - high magnesium hydroxide content contaminates the recovered lime.
6. Disposal Routes
- Sanitary sewer discharge. The most common route for small and medium plants. Requires an agreement with the receiving wastewater utility, and the utility will care about hydraulic load, solids load, and the effect of alum on its own phosphorus chemistry (which is usually beneficial) and on its digester (which is usually not).
- Landfill. Dewatered cake to a permitted solid waste landfill. Landfills typically require a paint filter liquids test pass - no free liquid - which sets the minimum cake solids target.
- Land application. Alum residuals applied to agricultural land supply no nutrients and can immobilize soil phosphorus, which is beneficial in over-fertilized watersheds but can induce phosphorus deficiency at high loading. Lime residuals are valuable as an agricultural liming agent.
- Monofill or on-site lagoon storage. Long-term storage in a dedicated, permitted cell.
- Beneficial reuse. Alum residuals in cement and brick manufacture, in constructed wetland media for phosphorus capture, and as topsoil amendment.
- Direct surface water discharge. Requires a VPDES permit from DEQ, with limits typically on TSS, pH, aluminum or iron, and often a prohibition on solids discharge. This route has become uncommon.
Membrane concentrate and spent brine
These streams cannot be dewatered - the contaminant is dissolved. Disposal options are limited to sanitary sewer discharge (subject to the receiving plant's chloride and TDS tolerance), permitted surface water discharge with dilution modeling, deep well injection, evaporation ponds (only in arid climates, not Virginia), or zero-liquid-discharge crystallization, which is very expensive.
A 4.0 MGD conventional plant treats raw water at 25 NTU with a site-specific conversion of 1.2 mg/L TSS per NTU, doses 40 mg/L alum, and adds no polymer or powdered carbon. Approximately how many pounds per day of dry sludge solids are produced?
Under the Filter Backwash Recycling Rule, where must recycled filter backwash water and dewatering liquids be returned in a conventional filtration plant?
Why does alum sludge dewater far more poorly than lime softening sludge, and what conditioning method uniquely and permanently improves alum sludge dewaterability?