7.3 Water Treatment Residuals: Sludge, Backwash Water and Membrane Concentrate

Types of Residuals

OHA's Table 51 even awards classification points for residuals handling. Examples are 5 points each for discharge to lagoons, land application or mechanical dewatering, and 10 for discharge to a raw water source.

Estimating Solids Production

A common estimate for alum coagulation is:

$$\text{Dry solids (lb/day)} = 8.34 \times Q_{\text{MGD}} \times \left(0.44 \times \text{Alum dose} + \text{SS removed} + \text{Other additives}\right)$$

Doses are in mg/L. Raw-water suspended solids are often approximated as 1 to 2 times the raw turbidity in NTU.

Worked example. A 4.0 MGD plant doses 25 mg/L alum and adds 3 mg/L PAC. Raw suspended solids average 12 mg/L.

• 0.44 × 25 = 11 mg/L of alum floc
• Total = 11 + 12 + 3 = 26 mg/L of solids
• Dry solids = 8.34 × 4.0 × 26 = 867 lb/day

At 1 percent solids, that is about 867 ÷ (8.34 × 0.01) ≈ 10,400 gallons per day of sludge. Thickening to 4 percent cuts the volume by about 75 percent.

Backwash Water Management

• Equalization: backwash water arrives in short, large surges, so it goes to a washwater recovery basin or lagoon where solids settle and flow can be metered back slowly.
• Recycling: under the federal Filter Backwash Recycling Rule, conventional and direct filtration plants that recycle SFBW, thickener supernatant or dewatering liquids must return them through all treatment processes, at or before the point of primary coagulant addition. Recycle flow must be limited so it does not overload the plant, and the practice must be documented and reported.
• Discharge to surface water: sending backwash or settled supernatant to a stream is a point-source discharge needing DEQ coverage. DEQ's 200-J NPDES general permit covers filter backwash; other cases need an individual permit. Chlorine residual and solids must meet permit limits, which often means dechlorination and settling.
• Discharge to sanitary sewer: allowed only with the receiving utility's approval and pretreatment conditions. Large alum slugs can upset a small wastewater plant.

Thickening and Dewatering

1. Gravity thickeners concentrate coagulant sludge, sometimes with polymer.
2. Lagoons settle solids over months. They are periodically decanted, allowed to dry, and the solids removed. Freeze-thaw cycles in colder parts of Oregon help dewater alum sludge.
3. Sand drying beds work for small plants in dry summer climates.
4. Mechanical dewatering (belt presses, centrifuges, screw presses, plate-and-frame presses) produces cake from about 15 to over 35 percent solids, depending on the sludge and equipment.

Disposal Routes

• Landfill: the cake must pass the paint filter liquids test, meaning no free liquid drains through a standard paint filter in 5 minutes.
• Land application: alum and ferric residuals can be land-applied with approval. They bind phosphorus, so agronomic suitability matters.
• Beneficial use: lime residuals for agricultural liming; alum residuals for soil amendment or phosphorus control.
• Concentrate disposal (RO and NF): sewer discharge, surface discharge under permit, deep-well injection where allowed, or evaporation ponds. Concentrate often decides whether RO is practical at all.

Operating Tips

• Record sludge volumes, solids percentages and polymer use so you can spot changes in raw water or coagulant performance.
• Do not let recycle streams carry high turbidity or pathogens back to the head of the plant during upsets. Divert or hold them.
• Treat residual handling areas as part of plant security and spill planning. Lagoon berms, decant structures and chemical cleaning wastes all need inspection.

Worked Example: Thickening Volume Reduction

Thickening removes water, not solids, so the dry solids mass stays the same while the volume shrinks.

Example. A plant withdraws 50,000 gallons of alum sludge at 1.0 percent solids from its basins and gravity-thickens it to 4.0 percent.

• Dry solids = 50,000 gal × 8.34 lb/gal × 0.010 ≈ 4,170 lb
• Thickened volume = 50,000 gal × (1.0 ÷ 4.0) = 12,500 gallons

Quadrupling the solids concentration cuts the volume to one quarter. That shrinks the hauling, lagoon space or dewatering capacity the plant needs. The 37,500 gallons of separated water become supernatant, which goes back to the head of the plant under the recycle rules or out under a discharge permit.

Residuals Lagoon Operation

Many small and medium Oregon plants send coagulant sludge and backwash solids to lined lagoons or drying beds:

• Fill and decant: route flow to one cell while another settles. Decant clear supernatant from the surface, and do not draw down so far that settled solids resuspend.
• Rest and dry: take a full cell out of service so solids can drain and dry before removal. In colder parts of the state, freeze-thaw cycles break up gelatinous alum sludge so it drains far better after thawing.
• Remove and haul: track solids depth with a sludge judge. Plan removal before solids reduce settling volume and carry over into the decant.
• Records: log volumes sent to each cell, decant volumes and quality, and quantities hauled, with destinations, for permit and sanitary survey review.