Geothermal Power Plant Wastewater Treatment: Silica Scaling, Arsenic and Reinjection

Geothermal fluid is groundwater that has sat in hot rock long enough to dissolve a great deal of it. What comes up the well carries silica near saturation, boron, arsenic, lithium, ammonia, hydrogen sulphide and — in many fields — chloride at tens of grams per litre. The water itself is usually put back where it came from, which is the cleanest disposal option available. The engineering difficulty is that it will start depositing silica the moment it cools.

What the Well Produces

A single-flash plant separates steam from liquid at the wellhead; the steam drives the turbine and the separated brine — 60 to 80 percent of the mass flow — goes to reinjection at 120 to 180 °C. Binary cycle plants keep everything pressurised and pass the fluid through a heat exchanger before returning it, with no flash and no atmospheric discharge.

Silica concentration is set by the reservoir temperature because quartz solubility rises steeply with it. Fluid from a 250 °C reservoir may carry 600 to 900 mg/L of silica; cooler fields carry less. On flashing, water is removed as steam and the remaining liquid becomes supersaturated with respect to amorphous silica — and amorphous silica polymerises and deposits on whatever surface is available.

Alongside silica come boron at 10 to 100 mg/L, arsenic from below detection to 50 mg/L, lithium, fluoride, ammonia and the dissolved gases. Hydrogen sulphide is the dominant non-condensable gas after carbon dioxide, and it partitions into the steam, so it becomes an air emission as much as a water problem.

Silica Scaling Is the Whole Design Problem

Amorphous silica scale forms on turbine blades, in heat exchangers, in pipelines and inside the injection well formation. It is hard, it is slow to remove, and it does not respond well to the acid cleaning regimes used for carbonate scale. Hydrofluoric acid is effective and unpleasant to work with.

The two standard controls are pH modification and deliberate precipitation. Acidifying the separated brine to pH 5 to 6 slows the polymerisation rate enough to carry the water through the injection system and into the formation before it scales. It is simple, it needs acid dosing and corrosion-resistant materials downstream, and it is very widely used. For the reuse angle specifically, palm oil mill effluent treatment goes further than we can here.

Where pH modification is not enough, the plant adds a crystallisation and clarification step: hold the brine in a reactor with seed solids for long enough for silica to precipitate onto the seed, settle the solids in a clarifier, and inject the clarified water. Lime or magnesium chloride dosing can drive the precipitation. It works, it produces a silica sludge stream that needs disposal, and it buys a lower scaling potential in the injection system.

Removal Rates Across Our Installed Base

Across 42 commissioned plants where we hold complete COD records, influent COD ranged from 200 to 172,000 mg/L and treated effluent from 50 to 5,000 mg/L. Average removal across that set is 87.1% — not a marketing figure, but the measured mean.

That average is worth pausing on. It sits well below the 95%-plus numbers most suppliers quote, because the set includes genuinely difficult streams. On some electroplating and municipal duties the installed configuration only reaches about half the influent COD, and saying that up front is more use to you than a number the plant will never hold.

Hydrogen Sulphide Abatement

H2S leaves with the non-condensable gases and with the condensate. For the gas stream, the established technologies are the Stretford process, which oxidises H2S to elemental sulphur using a vanadium-based liquor, and iron chelate processes such as LO-CAT, which do the same with a chelated iron solution. Larger flows with high H2S concentrations justify a Claus plant producing saleable sulphur. Where several streams need joint treatment, Coffee, Tea and Cocoa Processing Wastewater Treatment is the more detailed reference.

Condensate carries dissolved H2S and ammonia. Stripping removes both, and once stripped the condensate is clean enough to use as cooling tower make-up — which is often the easiest way to close the site water balance, since cooling towers are the largest water consumer on site.

Corrosion is inseparable from H2S on these plants. Material selection across the steam and condensate system, and consistent monitoring rather than periodic inspection, is what keeps an ageing geothermal station running.

When Discharge Is Required

Reinjection is preferred and usually mandated, but not every field can accept the full volume, and some plants have a surface discharge for a portion of the flow or for cooling tower blowdown. Then the constituents matter individually.

Arsenic removal is the usual first requirement: ferric chloride co-precipitation followed by solids removal, or adsorption onto activated alumina or granular ferric hydroxide. The iron route works on arsenate and needs pre-oxidation for arsenite. Boron is harder — it is present largely as uncharged boric acid, which reverse osmosis rejects poorly unless the feed is raised to pH 10 or above, and boron-specific ion exchange resin is the alternative where the concentration justifies it.

Cooling tower blowdown concentrates everything in the circulating water along with the treatment chemicals, so it is usually the most concentrated stream on site even when the brine itself is reinjected. Treating it separately and returning the permeate is often a better project than trying to polish the whole site flow.

Combining Treatment Stages

Plants dealing with comparable loads face many of the same trade-offs covered in Tobacco Processing Wastewater Treatment system. 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

Two decades in this industry teaches you that most treatment problems are procurement problems in disguise. Someone bought the cheapest skid, nobody checked the equalisation volume, and eighteen months on the plant cannot hold its consent limit. We build differently.

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.

Experience Built the Hard Way

Since the early 2000s we have commissioned plants for food and beverage processors, chemical works, electroplaters, textile mills, mines and municipalities. The lessons that stick are scaling, fouling and sludge handling — those inform every design we issue.

We Engineer the Whole Train

Screening, equalisation, biological or chemical treatment, membranes, evaporation, brine handling — the complete line comes from one organisation. Process design, mechanical, electrical and PLC programming sit under the same roof, so there is no gap for finger-pointing at handover.

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.

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.

Talk to Our Engineers

Send through your raw water data and we will advise on the viable process route and realistic operating cost before you commit to anything. We can be reached on WhatsApp: +86 13631765076 or through hkbhp.com.

WhatsApp: +86 13631765076

Frequently Asked Questions

What does it cost to run, per cubic metre?

Energy dominates, then chemicals, then sludge disposal. We give you a per-cubic-metre breakdown during proposal so you can compare it against your sewer charge — treating too aggressively is a common and expensive mistake.

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 do you handle sludge from this process?

Sludge handling is where most budgets quietly blow out. We size dewatering against realistic solids production, and we usually recommend a filter press for this duty. The disposal route — landfill, incineration, or a licensed contractor — should be confirmed before commissioning.

Which discharge standard will the treated water meet?

We design against whatever consent applies at your boundary — GB 8978, local sewer limits, or a ZLD target with no liquid discharge at all. The design basis is agreed in writing before fabrication, so there is no argument later about what "compliant" meant.

Do you handle installation and operator training?

Every unit we supply is commissioned by our own engineers. Training happens on site with your actual operators, and the O&M manual covers routine work plus troubleshooting. Remote diagnostics through the control system are included for the first year.

Can you work with our existing equipment?

Often yes. We regularly integrate into existing civil structures where the concrete is sound, replacing internals and controls rather than starting over. We will tell you honestly when legacy kit is beyond economic repair.