An industrial wastewater audit establishes how much water your site takes in, where it goes, what each wastewater stream contains and how that changes from one operating state to the next. It ends with a closed water balance, characterised streams and a ranked list of actions. That data pack is what a regulator, a sewer operator and a treatment supplier will each ask you for, usually in a different format and at a different time.
Brine Consulting is an independent industrial water advisory firm in Amsterdam. We sell no treatment equipment, so an audit from us is not the first step of a plant sale: if segregating two streams or recalibrating a meter solves the problem, that is the finding. We audit sites in the Netherlands and the EU directly, and support sites elsewhere through a defined engagement in which sampling is carried out locally and analysed by an accredited laboratory. The boundary and the sampling calendar are agreed on a short scoping call before any work starts.
When is an audit the right first step?
Commission an audit when the question is still “what do we actually have?” Typical triggers:
The water balance does not close. Metered intake and metered discharge differ by more than measurement error, and nobody can say where the difference goes.
Discharge charges keep rising. Sewer surcharges or water board levies are calculated from sampling or tables you have never checked against your own data.
A permit is up for renewal or a new limit is coming. Under the EU Industrial Emissions Directive, some sector BAT conclusions expect an inventory of waste water streams with flows, variability and loads (more on this below).
You are planning reuse, MLD or ZLD. Suppliers are asking for flows and analyses you do not have, or you have one grab sample from a quiet week.
An existing treatment plant underperforms. The cause is often the feed: a cleaning cycle, a batch dump or a stream that was added after the plant was designed.
Audit, treatability study or feasibility study?
These three pieces of work are easy to confuse, and buying them in the wrong order means paying twice for the same data.
| Wastewater audit | Treatability study | ZLD or MLD feasibility study | |
|---|---|---|---|
| Question it answers | What streams do we have, how much, how variable, and what can we avoid, segregate or reuse? | Will a given treatment step work on this water, and at what dose, recovery or flux? | Is zero or minimal liquid discharge achievable and economic, and which treatment train fits? |
| Main work | Desk review, site walk, water balance, sampling by operating state | Bench or pilot tests on real samples | Mass balance, technology screening, cost model |
| Main output | Closed water balance, stream inventory with variability, action list | Test results and design parameters for one or two technologies | Ranked options with capex, opex and cost per m3 |
| Comes | First | After the audit, once the stream to be treated is defined | After the audit, often alongside or before testing |
If your streams are already metered, sampled across operating states and reconciled, you may not need an audit; go straight to our ZLD feasibility study guidance or to testing.
What the audit covers
Desk review. Meter records and calibration certificates, production logs, cleaning schedules, existing lab results, discharge permits, levy or surcharge statements and drawings. This shows which numbers are measured, which are estimated and which are missing.
Site walk. Each stream is traced from where it is generated to where it leaves the site, with attention to unmetered connections, intermittent discharges such as tank drains, filter backwashes and clean-in-place, and points where clean and contaminated water mix.
Water balance and closure. Inputs, outputs, internal recycles and storage changes are put into one ledger and closed. Any remaining error is assigned to a cause rather than written off as “losses”.
Sampling by operating state. The sampling plan follows how the site really runs: normal production, cleaning, start-up, campaigns and seasons. Flow-proportional composites are used where load matters, grab samples for short events and unstable parameters. Samples are analysed by an accredited laboratory, and every full analysis is checked for charge balance and consistency with measured TDS before it is used.
Findings and actions. A ranked list of measures: what to eliminate, what to keep separate, what can be reused directly, what needs treatment and what the residual stream to a future treatment plant really looks like.
One dataset, three audiences
The same flows and analyses get requested three times: by the permitting authority, by whoever bills you for discharge, and by whoever designs your treatment. Each wants something slightly different.
| Who asks | What they ask for | What the audit should produce |
|---|---|---|
| EU permitting authority, chemical sector | The common waste water and waste gas BAT conclusions (2016) set BAT 2: an inventory of waste water streams with average values and variability of flow, pH, temperature and conductivity; concentrations and loads of relevant pollutants such as COD or TOC, nitrogen, phosphorus, metals and salts; and biodegradability data. BAT 3 adds continuous monitoring of flow, pH and temperature at key locations. | A stream inventory in that structure, with variability stated, not just averages |
| EU permitting authority, food, drink and milk | The food, drink and milk BAT conclusions (2019) ask, in BAT 2, for information on water use “e.g. flow diagrams and water mass balances” and on waste water flow, pH, temperature, TOC or COD, nitrogen, phosphorus, chloride and conductivity, with their variability. | The water balance and stream table, kept up to date when the process changes |
| Sewer operator or water board | Where an operator charges on measured strength, its own sampling sets your bill. Greater Cincinnati’s sewer district, for example, bases surcharges on BOD, TSS and TKN results from sampling that generally runs five days, flow-proportional where feasible, and gives users 30 days from the surcharge letter to request re-sampling (MSDGC policy, 2014). In the Netherlands, dischargers below 1,000 pollution units are usually assessed from a coefficient table, and a company can measure and sample to show it belongs in a different class; the burden of proof is on the company (Dutch government guidance, IPLO). | Your own representative sampling, on the parameters the bill is based on, ready before the next assessment |
| Treatment designer or supplier | Flow cases (normal, design, peak, minimum), full ion analysis, temperature, organics, scaling species and how they change with operating state | A design basis that every supplier receives in the same form |
In our view, a site that samples once for its permit, again to argue about a surcharge and a third time for a supplier is likely to end up with three data sets that do not agree. One audit designed with all three users in mind gives one set that does.
A worked example: what a balance that does not close tells you
This example is hypothetical, with round numbers chosen to show the method. It uses the same closure arithmetic as our free Industrial Water Balance tool.
A plant meters 28.0 m3/h of mains water and 12.0 m3/h of groundwater, so 40.0 m3/h comes in. On the output side it has 27.5 m3/h on the sewer meter, an estimated 6.0 m3/h of cooling tower evaporation, 1.2 m3/h leaving in product and 0.3 m3/h in sludge cake. Tank levels rose over the period, equal to 0.8 m3/h.
Closure error = inputs minus outputs minus storage change = 40.0 minus 35.0 minus 0.8 = 4.2 m3/h. That is 10.5% of intake, or about 36,800 m3 a year on continuous operation.
The audit does not spread that 4.2 m3/h across the other lines. It tests the likely causes one by one:
Sewer meter. Is it calibrated, sized for the flow range, and installed where it sees all the discharge? An under-reading meter means the site discharges more than it reports, and any treatment plant sized on 27.5 m3/h is too small.
Cooling tower evaporation. Was it measured or assumed? Make-up and blowdown meters, or the conductivity ratio between them, give an independent figure.
Unmetered outlets. Stormwater drains that also take washdown, tanker loading, a second connection to the sewer.
Leaks. Buried mains and fire water rings, checked against night-time or shutdown flow.
Each finding changes something real: the size of a future plant, the discharge you declare, or the water you pay for and lose.
What you receive
A closed water balance for the agreed boundary, with each flow labelled as measured, calculated or estimated, and the remaining closure error stated
A stream inventory: flow cases, composition by operating state and loads, with the sampling method and laboratory behind each result
A segregation and reuse assessment: which streams should stay separate, which can be reused directly and which should go to treatment
A ranked action list, separating no-cost operating changes from items that need investment or further testing
A defined design basis for any next step, whether that is a treatability study, a feasibility study or a request for proposals
Where relevant, the stream inventory set out to support a BAT conclusions review or a discussion with your sewer operator or water board
What drives the scope
An audit is quoted per site after a scoping call, because the effort depends on the site rather than a standard package. The main drivers are:
Number of streams and discharge points. A single process line with one outfall is a different job from a multi-product site with several sewers.
Metering. Where meters are missing, temporary flow measurement or tank-level methods add time.
Operating states. Sites that run campaigns, seasonal products or periodic cleaning need sampling during those events, which sets the calendar.
Analytical scope. A full ion balance, metals and priority substances cost more than the parameters on a discharge permit.
Regulatory output. Whether the inventory must be structured for a BAT review or a levy assessment.
How to prepare
If you can send these before the call, the scoping is faster and the audit itself starts further ahead:
A site plan or block flow diagram, even a hand-drawn one
Twelve months of intake and discharge meter readings
Your discharge permit and the last year of levy or surcharge statements
Any existing lab results for wastewater streams
A list of cleaning, regeneration and batch activities with their frequency
You can also start the balance yourself in the Industrial Water Balance tool. It shows the closure error and recycle ratio, and a share link keeps your inputs so we can look at the same numbers on the call. If you already have analyses, run them through the Water Analysis Validator to see whether the ion balance holds up.
Questions plant teams ask
How long does an audit take? It depends mostly on how long it takes to see every relevant operating state. A site with steady production can be balanced and sampled quickly; one that runs campaigns may need sampling spread over several of them. The Texas Water Development Board’s best management practice for industrial water audits (2004) says very large or complex audits should be completed within the first 12 months of adopting the practice (TWDB BMP 2.3).
What savings should we expect? That depends on what has already been done. The same TWDB guidance (Texas, 2004) puts water savings at 10% to 35% where no similar audit has been carried out before. Treat that as a published planning range, not a promise for your site.
Is this the same as a compliance audit? No. A compliance audit checks your records and practices against your permit. A wastewater audit is an engineering exercise: it measures and reconciles what the site discharges. It uses the permit as one input and often produces the evidence a compliance review needs.
Can we do it ourselves? Much of it, yes, especially the balance. Outside help earns its fee where the closure error will not go away, where sampling has to be designed around events, or where the results will be used to defend a levy assessment or size a plant.
Do you only audit sites that are heading for ZLD? No. Segregation, reuse and operating changes identified in an audit can make a large treatment project unnecessary. Where a high-salinity stream remains, our water treatment and ZLD consulting picks up from the audit’s design basis. For the regulatory side, see our posts on IED 2.0 water requirements and industrial discharge permits in the Netherlands.
Book an audit scoping call
Start with the boundary: which part of the site, which discharge points and which operating states. On a short call we agree those, the sampling calendar they imply and the deliverables you would receive. Book an audit scoping call, or send us your meter readings and discharge permit first so the call can start from your numbers.
Written by the Brine Consulting advisory team. The Brine Consulting advisory practice includes Osis G. Kalache, P.Eng., Senior Consultant, Wastewater Process Engineering & Optimization, with more than 17 years of experience in industrial and conventional wastewater treatment and process optimization, and Dimitra Aravani, External Advisor, Industrial Wastewater & Water Reuse, with more than a decade of experience delivering water efficiency, reuse and resource recovery projects. Meet our experts.