3.5 Water Treatment Residuals: Sludge, Backwash Recycle & Plant Discharges

3.5 Water Treatment Residuals: Sludge, Backwash Recycle & Plant Discharges

Treating water concentrates what is removed into residuals. The WPI Water Treatment Class 1, 2 and 4 outlines list "residuals disposal (lagoons, sludge drying beds, land application, on-site disposal, solids composting)" under treatment process, and the administrative area adds "monitor and control residual effluents to comply with regulatory permit limits." Poor residuals handling causes recycle upsets, permit violations and lost filter capacity.

Types of Water Plant Residuals

The Filter Backwash Recycling Rule (FBRR)

Recycling backwash water can return concentrated Cryptosporidium oocysts to the plant. The Filter Backwash Recycling Rule, 40 CFR 141.76, applies to surface water and GWUDI systems using conventional or direct filtration that recycle spent filter backwash water, thickener supernatant or liquids from dewatering:

• Recycle flows must be returned through all the processes of the plant's conventional or direct filtration system, which in practice means at or before the point of primary coagulant addition, unless the state approves an alternate location.
• The system must report its recycle practices to the state, including a plant schematic showing recycle flows and the return location, and keep records of recycle flow and frequency.

Good operating practice is to collect backwash in a washwater recovery basin, settle it, and return only the clarified decant slowly and steadily. Design standards commonly limit recycle to a small fraction of raw-water flow (on the order of 10 percent) so that recycled solids and pathogens do not overwhelm coagulation. Sudden slugs of recycle are a common cause of settled-water turbidity spikes.

Thickening and Dewatering

1. Gravity thickening with polymer concentrates clarifier sludge before dewatering; supernatant returns to the head of the plant under the FBRR.
2. Lagoons hold sludge for settling and drying; decant is returned or discharged under permit.
3. Sand drying beds use drainage and evaporation; alum sludge can crack and dry well in the Alabama summer but slowly in wet seasons.
4. Mechanical dewatering uses belt presses, centrifuges or plate-and-frame presses. Alum and ferric sludges dewater poorly without polymer, while lime sludge dewaters readily.

Disposal Options

• Landfill: residuals must pass the paint filter liquids test (EPA Method 9095) and the landfill's acceptance criteria.
• Land application: some residuals can be applied to land; aluminum residuals bind phosphorus, which can be a benefit or a limit depending on soil needs.
• Sanitary sewer discharge: requires the receiving wastewater utility's approval and compliance with its pretreatment limits.
• Surface water discharge: discharging backwash water, lagoon decant or other plant wastewater to a stream is a point-source discharge that needs an NPDES permit from ADEM, with limits that commonly include TSS, pH, total residual chlorine and metals such as aluminum or iron.
• Membrane concentrate: sewer, permitted surface discharge or deep-well injection, depending on volume and quality.

Estimating Sludge Production

A widely used estimate for alum plants (with alum reported as the commercial 14-hydrate product) is:

$$S = 8.34 \times Q \times (0.44 \times \text{Al} + \text{SS} + A)$$

where $S$ is dry sludge in lb/day, $Q$ is flow in MGD, Al is the alum dose in mg/L, SS is raw suspended solids removed in mg/L (often estimated from turbidity with a site-specific factor) and $A$ is other additives such as polymer or powdered carbon in mg/L.

Worked example. A 5.0 MGD plant doses 30 mg/L alum; raw suspended solids removed are estimated at 15 mg/L, with no other additives.

$$S = 8.34 \times 5.0 \times (0.44 \times 30 + 15) = 41.7 \times 28.2 = 1{,}176\text{ lb/day}$$

If clarifier sludge is withdrawn at 1.0 percent solids, the daily sludge volume is:

$$\text{Volume} = \frac{1{,}176\text{ lb/day}}{0.010 \times 8.34\text{ lb/gal}} = 14{,}100\text{ gal/day}$$

Operating Checklist

• Blow down clarifier sludge often enough to prevent septic, floating sludge but not so long that thin water is wasted.
• Return recycle water at a steady, limited rate and only after settling.
• Track backwash water use as a percent of production; rising use means shorter filter runs.
• Sample any permitted discharge as the permit requires and report it on the Discharge Monitoring Report.
• Neutralize spent membrane cleaning solutions before disposal.