RECD for Water Treatment Plants: Humidity and Corrosion Planning

Water treatment plants are wet places, and not only in the obvious way. Clarifiers and filter beds release moisture into the air all day. Chlorine buildings leak traces of a gas that eats copper and pits steel. Sewage treatment plants add hydrogen sulphide, which blackens connectors and corrodes electronics long before anyone smells a problem. Then the monsoon arrives and soaks everything that was not already damp.

A diesel generator at such a site works hard when it is needed, because pumps and blowers are heavy loads. That is good news for the particulate filter inside a retrofit emission control device. The challenge for an RECD for water treatment plants is not the duty. It is the air the device lives in, and what that air does to steel, insulation, fasteners, sensors and circuit boards over years of service.

This guide is organised as an exposure audit. You classify the atmosphere around your DG, match materials and protection to that class, deal with the moisture that forms inside the exhaust itself, and set up a seasonal inspection calendar.

Why Water Utilities Are Looking at RECDs

Water and sewage plants in Indian cities rely on DG sets to keep raw water intake, clear water pumping, booster stations and aeration running during grid failures. Municipal bodies, development authorities, industrial estates, large housing societies and private operators under PPP contracts all run such sets, ranging from 62.5 kVA at a small STP to well above 1000 kVA at a large water works.

In NCR, CAQM Direction No. 76 set out a route for each size class: dual fuel for 19 to 125 kW, a choice of dual fuel or RECD for 125 to 800 kW, and an RECD above 800 kW. Water pumping has generally been treated as an essential use under GRAP. In October 2023, for example, it was among the uses reported as allowed to run DG sets, as covered by India Gazette’s report on DG sets for emergency services. Exemptions and timelines change, so check the current CAQM direction and SPCB requirements for your plant. Our page on DG set emission regulations gives a plain summary.

Because water plants run their DGs during the very periods when air quality is worst, cleaner exhaust has a public health value beyond compliance. A water works is a public institution, and visible black smoke from its stack during a winter smog episode is not a good look.

What the DG Protects at a Water or Sewage Plant

Knowing the loads helps in two ways: it tells you how critical the set is, and it tells you how the filter will behave.

Plant type	Typical DG loads	Typical load level on DG
Water treatment plant	Raw water pumps, clear water pumps, backwash pumps, chemical dosing, chlorinators, lighting	Moderate to high
Booster pumping station	Booster pumps, controls	High when running
Sewage treatment plant	Inlet pumps, screens, aeration blowers, sludge pumps, dewatering	High; blowers are large continuous loads
Common effluent treatment plant	Similar to STP plus chemical dosing	Moderate to high
Building or campus STP	Small blowers and pumps	Often light relative to set size

Large pumps and aeration blowers keep the engine working hard, which means hot exhaust and good passive regeneration of the diesel particulate filter. Small building STPs are the exception: their DGs are often shared with other loads or oversized, and they may run light. Our guide on generator load factor and duty cycle explains why load matters for the filter.

Some loads are safety-critical. At plants that store chlorine in tonners or cylinders, the DG may back up chlorine leak detectors, alarms and the emergency neutralisation or scrubber system. Never plan DG downtime without first confirming how these systems will be powered.

Step 1: Classify the Atmosphere Around Your DG

Walk the plant and note where the DG sits relative to the main sources of moisture and corrosive gas. Then place the DG area in one of four exposure classes. A site can have different classes at the engine, the RECD position and the stack outlet, so classify each.

Class	Description	Typical sources	What it attacks
H: Humid, clean	High humidity and condensation, little corrosive gas	Clarifiers, filter galleries, cooling towers, monsoon	Carbon steel, insulation, electrical connections
C: Chlorine-affected	Traces of chlorine or chlorine compounds	Chlorinator rooms, tonner stores, hypochlorite dosing, leaks during cylinder changes	Copper, silver, steel, some stainless grades
S: Sulphide-affected	Hydrogen sulphide and other reduced sulphur gases	STP inlet works, wet wells, screens, grit chambers, sludge handling	Copper and silver in electronics, connectors, steel
M: Marine or saline	Salt-laden air	Coastal plants, desalination	Most metals, especially at joints

For electronics, the industrial standard often used to describe corrosive atmospheres is ISA-71.04, which grades environments from G1 (mild) through G2 and G3 to GX (severe), based on how fast copper and silver corrode. A plain explanation is available in this overview of ISA environmental classifications. If your plant has had control panels or PLCs fail early from corrosion, the DG area may well sit in the G2 or G3 range. Some utilities place copper and silver coupons near sensitive equipment to measure this directly; it is a low-cost test worth considering at sulphide-affected sites.

The Central Public Health and Environmental Engineering Organisation’s manuals, including the CPHEEO Manual on Sewerage and Sewage Treatment, discuss odour and corrosion control in sewage works and are a useful reference for the engineering team.

Quick location rules

• A DG building downwind of the STP inlet works usually belongs in class S.
• A DG room sharing a wall or ventilation path with the chlorinator building usually belongs in class C.
• A DG beside clarifiers or open filter beds, with no gas sources nearby, is usually class H.
• Any coastal plant within a few kilometres of the sea should assume class M on top of the others.

Step 2: Match Materials to the Class

An RECD is a steel housing containing ceramic catalyst and filter elements, with insulation, cladding, supports, fasteners, sensors, wiring and a display. Each part responds differently to a corrosive atmosphere. Discuss the following with your vendor during the survey and ask what options they can offer. The table is a guide, not a specification; your vendor and corrosion adviser should confirm the final choice.

Component	Benign site	Class H	Class C or S	Class M
Outer cladding	Aluminium or galvanised sheet	Stainless steel, sealed joints	Higher-grade stainless often preferred; seal all seams	Higher-grade stainless; regular washing
Supports and brackets	Painted mild steel	Hot-dip galvanised and painted	Galvanised with a suitable coating system	Galvanised with marine-grade coating
Fasteners	Zinc-plated	Hot-dip galvanised or stainless	Stainless, with isolating washers between dissimilar metals	Stainless, isolated, inspected often
Insulation	Standard mineral wool	Water-resistant insulation under sealed cladding	Same, with extra attention to seals	Same
Display and IoT unit	Standard enclosure	Sealed enclosure, cable entries from below	Locate in a clean control room if possible; sealed enclosure; ask about protective coating of boards	Same
Cable glands	Nylon	Sealed glands	Corrosion-resistant metal or suitable polymer	Same
Sensor tubing	Standard	Stainless	Stainless	Stainless

Three corrosion traps to avoid

Galvanic corrosion. When two different metals touch in the presence of moisture, the less noble one corrodes faster. Stainless fasteners on galvanised supports, or aluminium cladding fixed with steel screws, are common examples. Use isolating washers and sleeves.

Corrosion under insulation. If rain or condensation gets past damaged cladding, the insulation underneath stays wet for weeks, and the steel beneath it corrodes out of sight. Cladding seams should shed water outward, and damage must be repaired promptly.

Trapped water at supports. Flat-topped brackets, upturned channels and pockets in support frames hold rainwater. Specify drain holes and sloped surfaces.

Step 3: Manage Moisture Inside the Exhaust

Corrosion does not only come from outside. Diesel combustion produces water vapour, and fuel sulphur forms sulphur oxides. When a cold engine starts, the exhaust pipe is cool and some of that vapour condenses on its inner walls as a mildly acidic liquid. In a humid plant, where pipework rarely dries out fully between runs, this repeats every time the set starts.

The effects are slow but real:

• Internal corrosion of the exhaust pipe, especially at low points and joints
• Rust scale that can break loose and travel downstream
• Water collecting at the RECD inlet if the pipe slopes towards it

Design responses

1. Slope the pipework so condensate drains away from the RECD, not into it.
2. Fit condensate drains at low points, with a valve or trap that can be checked during routine visits.
3. Insulate the line so it warms faster after start-up, reducing the time spent below the dew point.
4. Use low-sulphur diesel. Our article on how ultra-low sulphur diesel improves emission control explains how fuel sulphur affects after-treatment.
5. Avoid frequent short runs. A short test run warms the engine just enough to fill the cold exhaust with condensate, then stops before the pipe dries. Longer, loaded runs are better.

Keep rain out of the stack

Rainwater falling down an exhaust stack into a hot filter element is a risk. A sudden splash of cold water on hot ceramic can cause thermal stress, and water soaking into the soot layer is unhelpful. Ask the vendor how the outlet is protected against rain ingress, for example with a drained stack design or a rain diverter that does not push the plume down onto the plant. Check that any flap or cap opens fully under exhaust flow and does not seize from corrosion.

Step 4: Protect the Electronics

The RECD’s sensors, wiring and display are the most corrosion-sensitive parts of the system. A filter that works perfectly is of little use if its backpressure sensor fails and nobody notices rising soot load.

Practical steps:

• Mount the display and IoT unit in the cleanest available location, such as a control room, MCC room with conditioned air, or a sealed panel away from the corrosive source.
• Keep cable entries at the bottom of enclosures, with drip loops on cables so water runs off before reaching the gland.
• Seal unused entries.
• Ask about protective coating of circuit boards for class C and S sites.
• Use breathers or desiccant in enclosures exposed to large day-night temperature swings, which pump humid air in and out.
• Inspect connectors at every visit for green or black tarnish, a sign of chlorine or sulphide attack.

Remote monitoring also helps here. If sensor readings become erratic, the IoT history will often show it before a visit does.

Step 5: Set Up a Seasonal Inspection Calendar

Water plants have a strong seasonal rhythm, and corrosion follows it. Build RECD checks into your plant’s existing preventive maintenance schedule along these lines. Frequencies are illustrative.

Season	Focus	Checks
Pre-monsoon (May to June)	Prepare for rain	Inspect cladding seams and repair damage; clear condensate drains; check stack rain protection; check enclosure seals and glands
Monsoon (July to September)	Water ingress	Monthly visual check for water at drains and under cladding; check display and sensor readings for drift; check supports for standing water
Post-monsoon (October)	Recovery and winter readiness	Look for rust staining at fasteners and joints; check insulation dryness; confirm everything is ready before GRAP season
Winter (November to February)	Condensation and heavy use	Watch for condensate during cold starts; review backpressure trends during long DG runs; check connectors after fog periods
Summer (March to April)	Heat and dust	Check radiator and air filter; review exhaust temperatures under peak pumping load
Every visit	Corrosive gas exposure	Check connectors for tarnish at class C and S sites

Our RECD maintenance guide covers the general technical checks, and the monthly maintenance checklist for diesel generators covers the engine side.

Installing the RECD at a Water or Sewage Plant

The fitment itself follows the usual sequence of survey, design, fabrication, fitment and commissioning, explained in our RECD installation process guide. DG downtime for fitment is usually a day or two per set. Water plants need some extra precautions.

Keep water flowing

• Schedule fitment when demand is lower, often outside peak summer.
• Confirm grid reliability for the window, and arrange a rental set if the plant cannot tolerate an outage.
• Coordinate with the distribution team so that reservoir levels are high before the work starts, giving some buffer if the grid fails.
• At STPs, check the inlet sump capacity. If pumps stop for long, sewage can back up into the network.

Protect chlorine safety systems

If the DG backs up chlorine leak detection, alarms or the neutralisation system, provide alternative backup for the fitment window. Agree the arrangement with the plant’s safety officer.

Hot work near chlorine and sulphide areas

• Keep cutting and welding well away from chlorine tonners and chlorine lines. Chlorine reacts vigorously with hot steel and is highly toxic.
• At STPs, check for hydrogen sulphide and methane with a gas detector before starting any hot work near wet wells, sludge areas or covered channels.
• Treat any pit, sump or covered channel near the work as a confined space.
• Issue a hot work permit, post a fire watch and keep extinguishers at hand.

Lifting in wet ground

Plant yards near clarifiers and sludge beds can be soft, especially after rain. Check the ground before bringing in a crane or hydra, and use load-spreading mats where needed.

Commissioning Checks for Humid and Corrosive Sites

Add these to the vendor’s standard commissioning tests:

• Visual check that cladding seams face downward and are sealed
• Confirmation that condensate drains work and are accessible
• Check that dissimilar metals are isolated at fasteners and supports
• Measured backpressure at full pumping or blower load, compared with the engine maker’s limit; our article on how exhaust backpressure affects a DG set explains why this matters
• Exhaust temperature at the device inlet under typical load
• Sensor and display readings compared with portable instruments
• Remote data path tested, with alarms reaching the duty engineer
• Photographs of all joints and connectors as a baseline for future corrosion comparison

The Load Advantage, and Where It Breaks Down

Most water and sewage plants have a strong advantage: large pumps and blowers keep the DG well loaded, so the filter regenerates passively during normal running. Two situations break that pattern.

Oversized sets at small STPs. Building and campus STPs often share a DG with other loads, or have a DG sized for future expansion. If the STP is the only load during an outage, the set may run light.

Short test runs. Many plants run DGs briefly once a week to prove they start. These short, light runs are bad for both condensate and filter regeneration. Replace them with periodic loaded runs that last long enough for the exhaust to heat fully.

For sets that run lightly or rarely, read whether an RECD suits a DG set that runs only occasionally.

Illustrative Plant Profiles

The three plants below are composites built to show the method; none is a real customer.

Municipal water treatment plant, 1 x 1010 kVA

A water works with raw and clear water pumping, a chlorination building and open clarifiers. The DG building sits between the clarifiers and the chlorine store.

Assessment: Class H at the engine, class C at the proposed outdoor RECD position near the chlorine building wall. At about 808 kW the set falls above 800 kW, the band where Direction 76 expected an RECD.

Plan: Move the RECD position to the far side of the DG building, away from the chlorine store. Stainless cladding with sealed seams, galvanised and coated supports, isolated fasteners. Display mounted in the conditioned MCC room. Fitment scheduled in February, after winter demand and before summer peaks, with reservoirs filled beforehand. The 1010 kVA DG set RECD is the reference size.

Sewage treatment plant, 2 x 500 kVA

Activated sludge plant with large aeration blowers. The DG house is downwind of the inlet screens and grit chamber.

Assessment: Class S throughout. Earlier failures of PLC cards in the DG panel suggest a severe atmosphere for electronics.

Plan: Copper and silver coupons placed for three months to grade the atmosphere. RECD electronics moved to a sealed, filtered panel room, with protective board coating requested. Monthly connector checks. Heavy blower load means good regeneration. One set fitted at a time. Specifications for this rating appear on the 500 kVA DG set RECD page.

Housing society STP, 1 x 125 kVA shared with common services

Small STP on a residential campus, with the DG also serving lifts and common lighting.

Assessment: Class S near the STP, class H elsewhere. At 125 kVA, roughly 100 kW, the set falls in the band where dual fuel was the stated route, so the society should first check which option applies using RECD vs dual fuel kit. Our guide on RECDs for housing societies covers the decision process for RWAs.

Questions for Your Vendor

1. What cladding, support and fastener materials do you offer for humid and corrosive sites?
2. How do you isolate dissimilar metals?
3. How is condensate managed, and where are the drain points?
4. How is the outlet protected against rain ingress without pushing the plume down?
5. Can the display and IoT unit be located remotely, in a cleaner room?
6. What enclosure rating and board protection are available?
7. What inspection schedule do you recommend for our exposure class?
8. What is the calculated backpressure for the proposed route?

Our guide to choosing CPCB approved RECD vendors covers certification checks.

Frequently Asked Questions

Will humidity damage an RECD at a water treatment plant?

Humidity alone mainly affects the outside of the system: cladding, insulation, fasteners and electronics. With suitable materials, sealed cladding and regular inspection, an RECD can serve well in a humid plant.

Is hydrogen sulphide at an STP a problem for the RECD?

It is mainly a problem for electronics and connectors, which can corrode quickly in sulphide-laden air. Locate the display and IoT unit in a cleaner room, use sealed enclosures and check connectors often.

What about chlorine near the DG set?

Chlorine attacks copper, steel and some stainless grades. Keep the RECD and its electronics away from chlorine buildings where possible, and protect them where not.

Why does water collect in our exhaust pipe?

Diesel exhaust contains water vapour that condenses in a cold pipe after start-up. Short runs and humid conditions make it worse. Slope the pipe, fit drains, insulate the line and run the set longer on load.

Do water plants need an RECD if water pumping is an essential service?

Essential-service exemptions under GRAP affect when a DG may run, not necessarily which emission controls it needs. Check the current CAQM direction and your SPCB for the requirement that applies to your set’s size.

How often should we inspect the RECD at a corrosive site?

Monthly visual checks are a sensible baseline at chlorine or sulphide-affected sites, with a fuller inspection before and after the monsoon.

Build for the Air It Will Breathe

At a water or sewage plant, the RECD will spend its whole life in damp, sometimes corrosive air. Classify the atmosphere, choose materials to match, keep water out of the exhaust and electronics, and inspect with the seasons. Aceget supplies CPCB type-approved RECDs for DG sets from 25 kVA to 2500 kVA, with IoT monitoring that helps you spot sensor drift and rising backpressure early. To discuss your plant, speak to the Aceget team or view the retrofit emission control device range.