In a tannery the effluent plant is the load that makes solar self-consume, and the atmosphere is what destroys the hardware. Corrosivity categories, the two IEC tests to demand of a module, and who has to consent.
On a tannery site the array is not the fragile part. The mounting, the fasteners, the combiner boxes, the connectors and the earth electrodes are, and they fail for reasons that have nothing to do with the sun. A leather processing plant puts hydrogen sulphide, ammonia and chloride into the air it sits under, and a solar plant specified to the default assumptions of a Dhaka warehouse roof will be visibly deteriorating well before it has paid for itself.
The good news is that the same plant has one of the better electrical load profiles in Bangladeshi industry for self-consuming solar, and it comes from the effluent treatment plant.
Tanning itself is intermittent. Drums turn for the soaking, liming, deliming, bating, pickling, tanning, retanning and dyeing stages, and between them the motors sit idle. Sammying, shaving, buffing, setting, toggling and spraying run on a batch rhythm. The boiler is usually gas or biomass, so the process heat is not electrical, and the plant's electrical demand swings with what is on the floor that day.
The effluent treatment plant does not swing. Aeration blowers run continuously because the biology cannot be stopped and restarted at the operator's convenience, and on most captive ETPs the blowers are the single largest continuous electrical consumer on the site. Around them sit influent and recirculation pumps, sludge pumps, dosing pumps, mixers, a filter press or screw press, and their controls. That is a flat, round-the-clock base load.
For sizing purposes, that base load is the anchor. Set the array so that midday output stays below the sum of the ETP base load and the daytime process load on an ordinary working day. Do that and almost every unit is consumed on site at full retail value. Oversize past it and the extra capacity is exported into whatever the net metering arrangement credits, which is a worse outcome for the same capital.
Units in the Savar estate that discharge to the central effluent treatment plant rather than running their own do not have that anchor, and their profile looks much more like a batch process factory. Size those against the measured process load, not against a generic industry figure.
This is worth setting out step by step, because the specification decisions follow directly from it.
| Process step | What it releases | What that attacks |
|---|---|---|
| Liming and unhairing with sulphide | Hydrogen sulphide, worst at drains, sumps and floor channels | Copper and silver: busbars, plated contacts, junction box internals, connector pins |
| Deliming with ammonium salts | Ammonia | Copper and brass fittings, and module encapsulants and frames that have not been tested for it |
| Pickling with salt and acid | Chlorides and acid mist | Stainless steel by pitting, zinc coatings, and aluminium under its coating |
| Chrome tanning, retanning and dyeing | Liquor mist and aerosol | Coatings generally, plus a film on glass that ordinary washing does not lift |
| Effluent treatment | Hydrogen sulphide and ammonia from tanks and sludge handling, continuously | All of the above, and worst within the footprint of the tanks |
Two consequences follow. The first is that the roof nearest the ETP is the harshest location on the site, and it is often the roof with the best solar access, because ETP buildings are low and unshaded. The second is that hydrogen sulphide attacks precisely the metals that carry current, which is why electrical enclosures and connectors matter more here than mounting steel does.
Corrosivity is a category, not an adjective, and there is a framework for it. ISO 9223 classifies atmospheric corrosivity, and ISO 12944 sets protective paint systems against those categories with a stated durability. A tannery roof, and particularly a roof near an ETP, will not be the C3 that a general industrial site in Bangladesh gets specified as by default. Assess it, write the category into the specification, and specify the coating system and its durability class against the assessed category.
Most module warranties exclude damage from a corrosive atmosphere. That exclusion is enforceable, and on a tannery it is the exclusion most likely to be invoked. The way around it is to specify modules that have been tested for the conditions and to get the test certificate before purchase, not the marketing sheet.
Connectors deserve their own paragraph. The single most common origin of a rooftop DC failure anywhere is a pair of connectors from two different manufacturers mated together, because the contact geometry and the tolerances do not match and the joint runs hot. In a sulphide-bearing atmosphere the same joint tarnishes as well. Specify one make of connector throughout the plant, crimped with that manufacturer's tool, and have the commissioning engineer record the crimp checks.
DC combiner boxes and inverters should be at IP65 as a minimum, with pressure-equalisation breather elements rather than open drain holes, because an open drain in a hydrogen sulphide atmosphere is an invitation. Ask the inverter manufacturer for a written statement of the corrosion class the unit is rated for and whether conformal coating on the boards is standard or optional on that model. Several manufacturers offer a coated variant; it is not usually what gets quoted unless it is asked for.
Site the inverter room upwind of the ETP and away from drain vents, and give it filtered ventilation. If the room has to sit close to the process, a sealed and slightly pressurised room with filtered supply is worth the cost, and the filter change interval goes into the maintenance schedule.
Earthing is the part most contractors under-think. A bare copper electrode driven into ground carrying chloride and chrome-bearing seepage will not last the way the design assumed. Use copper-bonded steel rods with an adequate copper thickness, put them in inspection pits so they can be seen and tested, and put an annual earth resistance measurement into the maintenance contract. An earth system that has quietly degraded is invisible until the day it matters.
Walk a tannery roof and look at the fastener holes and the gutter lines. On many sheds the sheet is already rusting through at the washers and along the valleys, because the same atmosphere that attacks the array has had a head start on the roof.
Putting a twenty-five year array onto a sheet with a few years left is a false economy of the most expensive kind, because removing and refitting an array to re-sheet a roof costs a large share of what the array cost to install, and it happens at a time nobody has budgeted for. If the sheet will not outlast the array, replace it first. That decision belongs in the feasibility stage, where it can be compared honestly against the return, rather than being discovered on the survey after a price has been agreed.
Cleaning water quality matters more here than most people expect. Washing glass with hard or contaminated water leaves a mineral film that is harder to remove than the deposit it replaced, and on a site where the deposit already includes liquor aerosol, the two combine. Specify a low-dissolved-solids supply for the final rinse.
Many tannery units occupy plots or sheds they do not own. Where the site sits inside an industrial estate, the estate authority's permission is a separate document from the landlord's, and both come before the utility file. Get the owner's written no-objection covering the roof area, the plant capacity, the mounting method and the term, and get the estate's approval for anything that alters the building envelope.
On the utility side, confirm which distribution licensee actually issued your connection before filling in anything. Savar and the surrounding area are not served by a single utility, and the net metering file goes to the one whose name is on your bill, under SREDA's guidance. Check the sanctioned load at the same time, because the guidelines cap the plant against it and a tannery's sanctioned load is often lower than the connected load its owners quote from memory.