Leachate Treatment Tanks Through the Equalisation Reaction Train
Created on 09.03
Leachate Treatment Tanks Through the Equalisation Reaction Train
Leachate treatment tanks cover several duties on the same site: raw leachate holding, equalisation, chemical dosing or neutralisation reaction, and treated effluent or concentrate holding. Each duty has a different residence time and a different chemistry, so each needs its own specification rather than one generic tank. Glass-fused-to-steel handles the raw and equalisation duties because the enamel is inert across pH 1–14; reaction vessels that need agitation or pH control should still be checked against the specific chemical and temperature. The design items that decide service life are the gas space treatment, the agitation that prevents settling, and the gasket material at every bolted joint.
A landfill treatment plant is a chain, and the tanks in it behave differently. Raw leachate arrives at a variable rate and a variable composition and has to be held. Equalisation blends the variation so the downstream process gets a feed it can treat. Neutralisation or chemical precipitation happens in a tank that is agitated and dosed. Then the treated effluent and any reverse-osmosis concentrate need holding before discharge or evaporation. Buying all of these as the same vessel because they are all "leachate tanks" is a common and expensive mistake. This article separates the duties, gives the design items for each, and explains what to specify so the chain runs without a lining failure in year three.
Duty by duty: what each tank actually does
Raw leachate holding is a storage problem; equalisation is a blending problem. The holding tank must simply cover the gap between arrival and treatment capacity at the worst generation rate, with enough freeboard to ride a weekend when the treatment line is down. Its design variables are volume, inlet arrangement that avoids disturbing the settled solids, and a level indication the plant control system can see. The equalisation tank does something subtler: it averages the composition so the neutralisation dose and the biological load downstream stop swinging. That means a longer retention, often a well-mixed or gently baffled configuration, and an outlet that draws from a blended zone rather than from the bottom where the solids sit.
Reaction and dosing tanks are the ones that need the most scrutiny, because they carry an added chemical that the original leachate analysis did not include. Acid or base for pH correction, coagulant and polymer for precipitation, and oxidant for iron or colour removal each bring their own compatibility question. The enamel envelope covers the whole pH 1–14 range across both ends, but a strong oxidant or a concentrated acid at elevated temperature is worth a documented material check rather than an assumption. Agitation in these tanks is not a convenience: without it, precipitates settle into a hard bed at the floor and the effective volume drops.
Lining choices and the gas space problem
The biggest corrosion risk in a leachate tank is not the liquid, it is the head space. Raw leachate sheds carbon dioxide, hydrogen sulphide and ammonia into the vapour space above the liquid line. Immersion-grade coatings often do poorly in that humid, sulphide-bearing atmosphere, and bare steel at the top course fails quickly. Glass-fused-to-steel solves the liquid side completely, and the practical answer for the gas side is to treat the roof and the vent as their own specification: a roof system with the correct corrosion allowance or coating, a vent sized for the gas generation, and where the roof is a gas-tight membrane option, a pressure and vacuum relief set at only a few kilopascals.
The alternative coating route is fusion-bonded epoxy, applied electrostatically and cured to a 180–280 µm film with more than 4000 hours of salt spray resistance under AWWA C550, which is a fused powder film rather than a paint and holds up well in immersion and splash zones. Hot-dip galvanising is not a candidate here; zinc is unsuitable in acidic and high-chloride service. Where the project leans to a welded stainless shell, 316L is the usual grade for chloride-bearing service and the field welds need non-destructive examination before the tank is commissioned, which is the reason many leachate projects choose a bolted enamel shell instead.
Agitation, settling and the settling solids
A leachate tank with no agitation becomes a sediment trap, and a sediment trap loses volume. Solids from raw leachate settle within hours. In a holding tank that is acceptable if the inlet is arranged so the settled layer is not disturbed and there is a dredge or a washout at the floor. In an equalisation tank it is not acceptable, because the whole point is to blend by mixing; the answer is a gentle air or mechanical agitation sized to keep the solids mobile without shearing the blanket into the effluent. A clarifier or a dissolved air flotation unit upstream changes the calculation entirely and usually makes the downstream tanks smaller.
For an anaerobic route on the settled fraction — and many leachate streams have a sloppy solids content that makes digestion plausible — the operating band is mesophilic at 35–38 °C with a retention of 20–30 days, and the gas is high in hydrogen sulphide. The enamel is inert to the sulphide in the liquid, but downstream boilers and gas handling need desulphurisation with FeCl₃ dosing or a dry iron oxide bed. That is a process decision, not a tank decision, but it belongs in the same design meeting because the tank gas space and the gas train are connected.
Technical Specification
Parameter | Typical Value / Range | Note |
Enamel fusion temperature | 820–930 °C | glass fused to steel in one cycle |
Enamel layer thickness | 0.25–0.45 mm | 1500 V DC spark test after assembly |
Enamel layer compressive strength | above 3,450 N/cm² | rated for the enamel layer |
pH resistance of enamel | 1–14 | covers acidic raw and alkaline treated leachate |
Interior surface roughness | Ra < 0.8 µm | lowers deposit adhesion on the wall |
Design life | at least 30 years | under the specified chemical service |
Panel width | about 1.2 m | bolted panel, ships flat to site |
Bolt grade | 8.8-grade bolts | corrosion-inhibited connection |
Gasket | EPDM, verify against pH and temperature | aging item in the inspection list |
FBE alternative | 180–280 µm, above 4000 h salt spray | AWWA C550 coating route |
Pressure class | atmospheric to low pressure | roof, vent and relief set the boundary |
Agitation | mechanical or gas, matched to settling rate | prevents sediment loss of volume |
Specifying the chain and what to hand the contractor
A workable specification lists each tank separately with its duty, volume, retention time, agitation requirement, inlet and outlet arrangement, and the fluid analysis with the expected variation. For each tank it names the lining and gasket, the roof and vent type with the relief setting, the level instrumentation, and the inspection access. It states the test protocol: the 1500 V DC spark test on the assembled shell for enamel, holiday detection for epoxy, and the agreed hydrostatic or level test before handover. And it names the applicable design reference, where that is AWWA D103-09 or AWWA C550 for the water and coating side and NFPA where fire or gas handling applies.
Where a project sits in a service class with no directly comparable delivery on file, we describe what is delivered in neighbouring duties for municipal and industrial wastewater, with the capacity range verified by project specification. That is a more useful statement than a plausible-sounding project that never happened, and it should be treated as such when comparing bidders.
Project Case
Project | Location | Product | Capacity | Scope |
Leachate tanks, two units | China, Beijing | GFS tanks | 9,682 m³ | φ16.23 × 23.4 m, supply and supervision |
Welded leachate tank | China, Hebei | welded steel tank | leachate service | supply and field welding |
Chemical wastewater tank | China, Zhejiang | GFS tank | 11,613 m³ | φ24.45 × 19.8 m, supply |
Chemical epoxy tanks, 11 units | China, Guangxi | fusion-bonded epoxy tanks | 4,771 m³ total | supply and supervision |
Center Enamel Engineering Capability
Center Enamel (Shijiazhuang Zhengzhong Technology Co., Ltd) has designed and fabricated bolted storage tanks since 2008. As the first glass-fused-to-steel (GFS) tank manufacturer in China, the company holds close to 200 enamel-related patents, produces roughly 300,000 enamel-coated steel plates a year, has completed more than 30,000 installed projects and supplies its tanks to over 100 countries. The new 150,000 m² production base was added to raise output capacity, and single tanks are supplied up to 60,000 m³. Manufacturing runs under ISO 9001 and ISO 45001, with product certification including NSF/ANSI 61, WRAS, FDA, LFGB, CE (EN 1090), ISO 28765, FM, BSCI and EUROCODE, and design referenced to AWWA D103-09, AWWA C550 and NFPA where the application requires it.
For leachate treatment the relevant scope is the holding and equalisation shells in glass-fused-to-steel, the roof and vent configuration for a sulphide-bearing head space, and the interface into an anaerobic digestion or precipitation train where the reaction chemistry has to be checked against the lining before the vessel is ordered.
Frequently Asked Questions
Q1: Which leachate duty should get a glass-fused-to-steel tank?
A1: Raw holding and equalisation, which is most of the chain, because the enamel is inert across pH 1–14. Reaction vessels need a documented check against the specific chemical added, and treated effluent or concentrate holding is normally within the same envelope.
Q2: Does a leachate tank need agitation?
A2: In an equalisation tank yes, or the tank becomes a clarifier and loses volume. In a raw holding tank a settled layer is acceptable if the inlet does not disturb it and there is a washout at the floor.
Q3: How do you handle the hydrogen sulphide in the head space?
A3: By treating the roof and vent as a separate specification from the lining: corrosion allowance or coating on the roof steel, a vent sized for the gas generation, and a gas-tight membrane roof with a pressure and vacuum relief where the site needs enclosure.
Q4: Can the tanks be bolted rather than welded?
A4: Yes, and on a landfill site it is often the better route because panels travel on standard transport to a remote plot, there is no field coating of the interior, and the bolted joints are inspectable in later years.
Q5: What gasket material suits leachate?
A5: EPDM is common for water and wastewater service, but the gasket must be verified against the specific pH, temperature and any chemical dosing, because the gasket ages in service even where the lining does not.
Q6: Is an anaerobic route realistic for leachate solids?
A6: Where the stream carries a digestible organic fraction, a mesophilic 35–38 °C digester with a 20–30 day retention is the standard approach, with desulphurisation of the gas before it reaches any boiler or genset.
Q7: What should be in the inspection programme?
A7: Bolt torque and gasket condition at the vertical joints, exterior coating, enamel appearance at the liquid line, the level gauge, the vent, and a spark retest after any panel that has been opened.
Leachate treatment tanks are only specified correctly one duty at a time. Hold, equalise, react and hold the product, each with its own volume, chemistry and agitation requirement, and with the roof, vent and gasket chosen alongside the lining rather than after it. Do that and the chain runs; skip it and the first lining failure will be in the tank nobody specified in detail.
Send us the treatment train, the leachate analysis with its variation, the flow and the retention you need at each stage, and we will return a tank-by-tank specification, the lining and gasket selection and the technical file package.
Talk to an Engineer
We will work through the train with you before quoting a single vessel. Share the flow, the chemistry at each stage and the plot, and you will get a duty-by-duty tank list, the material recommendation with its reasoning, and the drawings. Engineering input first, with no pressure to order.
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