Sand, Gravel and Aggregate Washing Water Treatment: TSS, Thickener and Filter Press Design

A washed sand and gravel plant moves a lot of water past a lot of silt. Throughputs of 100 to 400 m³/h are ordinary, and at 30 to 80 g/L suspended solids in the wash circuit, that is 3 to 30 tonnes of silt an hour coming off the product. Nothing about the chemistry is difficult. Everything about the mechanical handling is, and the plants that struggle are almost always the ones that sized the thickener on water flow rather than on solids flux.

The Load Is Solids, Not Contaminants

Aggregate wash water is a mineral suspension. Silt and clay below 75 µm, fine sand up to a few hundred microns, some organic matter from overburden, and occasionally a little clay that behaves badly — smectite and other swelling clays can hold water in a gel that no thickener will release at acceptable density.

Chemically there is usually nothing to remove. pH is near neutral unless the source is a limestone or a chalk quarry, in which case it sits at 8 to 9. Consents are almost always about suspended solids and turbidity, sometimes about pH and very occasionally about naturally elevated metals or sulphate from the geological formation.

Which means the treatment objective is purely physical: separate the solids, get the water back, and handle the sludge. Where the site also has a consent on turbidity below 10 NTU — increasingly common near sensitive watercourses — the polishing requirement gets tighter than the settling requirement.

Sizing on Solids Flux

The standard error is to size a thickener by rise rate. For a conventional high-rate unit on aggregate silt, a rise rate of 1.5 to 3 m/h is typical, and that gives an area. But the real constraint is often the solids loading — 0.5 to 1.5 t/m²·day for well-flocculated silt, lower for clay-dominated feed.

A thickener that is adequate in area but overloaded in flux produces a muddy underflow and a dirty overflow, and no amount of extra flocculant fixes it. Ask the thickener supplier for a flux curve based on actual flocculated settling tests on the site's own material. Jar tests are not sufficient; a proper dynamic test with a settling column is worth the small cost.

Underflow density is the number operators care about. Well-run aggregate thickeners produce 40 to 55 percent solids by weight from a silt feed, and 25 to 35 percent where there is significant clay. That difference decides whether the downstream filter press runs a 20-minute cycle or a 60-minute one.

Flocculant: Cheap, and Easy to Get Wrong

High molecular weight anionic polyacrylamide at 5 to 30 g per tonne of dry solids is the norm. Dosing looks trivial and the consequences of getting it wrong are not. Under-dosing gives a dirty overflow and slow settling. Over-dosing restabilises the fines, blinding the filter cloth, and produces a slimy underflow that will not dewater.

Make-up and ageing are where plants lose performance. Emulsion polymers need 30 to 60 minutes of maturation at the correct concentration — typically 0.1 to 0.5 percent — and they lose effectiveness if sheared by a centrifugal pump or stored too long in hot weather. A make-up unit with proper low-shear mixing and a dosing point that gives gentle, distributed contact into the feed launder is worth more than a marginal upgrade in polymer grade. Much of this mirrors the situation set out in Rare Earth Mining and Refining Wastewater Treatment.

Where the silt is very fine or the clay content is high, a coagulant ahead of the flocculant helps: 20 to 100 mg/L of a ferric or aluminium salt neutralises the clay surface charge and lets the polymer build a floc that will actually settle. The sludge volume goes up. The overflow clarity usually improves by a factor of several.

What This Looks Like in Practice

One reference point: a 5 m³/day installation we delivered in Taishan, Guangdong (2021) was treating feed at roughly 1,000 mg/L COD and holding discharge near 50 mg/L, about 95% removal.

Figures are site-measured values from commissioned plants, not modelled estimates.

Filter presses are the usual choice for aggregate silt because they produce a stackable cake at 60 to 75 percent solids, which can go to a licensed tip, to site restoration, or occasionally to a blending operation. Cycle times of 30 to 90 minutes are typical for a membrane press on this material.

Belt presses and centrifuges appear in some designs. They run continuously and need less operator attention, and they produce a wetter cake — 40 to 55 percent — which may not be handleable or acceptable for disposal. On a high-volume site the difference in cake moisture translates directly into lorry movements.

Water recovery is the economic driver. A well-run circuit returns 85 to 95 percent of wash water to the plant. The balance goes out with the cake and with evaporation. Closing further requires treating the dissolved load, which on most quarries means little more than monitoring — unless the source is a chalk aquifer, in which case hardness builds in the loop and a softening side-stream becomes necessary.

Consent, Runoff and the Rainy Day

Aggregate sites are open to the weather, and the treatment system has to deal with the fact that a 20 mm rain event puts the whole yard into suspension. Most consents allow a higher turbidity during defined storm conditions, but the design still needs a route for the extra volume: a lagoon or a settlement pond sized for a 1-in-10-year storm plus the operating inventory, and a way to return that water to the circuit once it settles. If your site also runs this kind of duty, Fibreglass and Mineral Wool Wastewater Treatment covers the cost side of the decision in more depth.

Oil is the other thing to plan for. Plant refuelling, hydraulic hose failures, and the general reality of a diesel-powered fleet put hydrocarbons on site. A bypass oil separator on the yard drainage and on the workshop area — before it reaches the silt lagoon — costs very little and prevents an oily sludge that no tip will accept.

Finally, keep the product stockpile drainage out of the wash circuit where possible. It is high volume and low solids, and routing it through the thickener just uses up hydraulic capacity for no benefit. Where several streams need joint treatment, Airport Deicing Runoff Treatment is the more detailed reference.

Combining Treatment Stages

We have written about the downstream implications separately in Caprolactam, PTA and Spinning Finish. Most facilities do not run a single clean stream, and a shared equalisation and biological stage is usually the economical answer once the streams are chemically compatible.

Why Choose Baihuipu as Your Manufacturer

Choosing a manufacturer here comes down to one question: who is still accountable once the commissioning team flies home. Our answer is that we build and programme our own equipment, then stand behind it for the life of the plant.

In-House Fabrication

Our Guangdong works produces everything from standard modular units to fully bespoke lines. Building in-house keeps cost, schedule and quality under our own control rather than a subcontractor's goodwill.

Documentation You Can Actually Audit

Each system ships with as-built drawings, material certificates, welding records and a functional specification. If a client's own consultant wants to review the process calculation basis, we hand it over rather than treating it as proprietary.

Certification and Export Practice

CE marking is standard and we work to ISO 9001 procedures. Where a project calls for specific material grades, pressure vessel certification or ATEX-rated equipment, we build to that standard and ship the full documentation pack.

Two Decades of Field Data

Food and beverage, chemicals, electroplating, textile dyeing, mining, municipal work — we have commissioned across all of them. The useful part is not the project count, it is having seen how plants fail in year seven and designing those failures out.

Twenty-Plus Export Markets

We have shipped to the United States, Canada, Russia, Indonesia, India, Nigeria and Vietnam among others. Crating, export documentation and certificates are handled in-house, which keeps customs surprises to a minimum.

Complete Units and Turnkey Plants

Whether you need a single package unit or a turnkey installation with civil works, we deliver the entire treatment scheme. Our engineering team covers process, mechanical, electrical and automation so one party owns the outcome.

Spares, Service and Commissioning

Membranes, dosing pumps, diffusers, instrumentation and blowers are stocked for the systems we sell. Our control systems support remote diagnostics, and we can put an engineer on site for commissioning, training or an unplanned shutdown.

Let's Look at Your Numbers

Share your water analysis and site constraints and we will give you a straight assessment of the process route and the realistic operating cost before you commit to anything. Contact our engineers at WhatsApp: +86 13631765076, or find us at hkbhp.com.

WhatsApp: +86 13631765076

Frequently Asked Questions

What information do you need to quote?

Water analysis for your worst-case stream, peak and average flows, discharge limits, available footprint and utilities. If you do not have a full analysis yet, we can start from a partial one and flag the gaps.

What spare parts should we hold on site?

Critical spares are pumps, dosing valves, instrumentation sensors and any specialty media. We ship a recommended spares kit with the plant and hold stock for everything we supply, so lead times on replacements stay short.

How much operator attention does the system need day to day?

Automatic control handles dosing, backwash and level control. Realistically your team spends time on chemical top-up, sample testing and sludge handling — maybe one to two hours per shift on aggregate wash water duty, less if you opt for remote monitoring.