A reclaimed water reuse project in a coastal industrial city – the project manager once likened the system to a set of precision multi‑stage sieves: from ultrafiltration membranes to reverse osmosis membranes, the pore sizes gradually decrease, with the finest being only 0.1 nm, tens of thousands of times thinner than a human hair.
The analogy is vivid, but it tells only half the story. What really makes a reclaimed water treatment process for industrial use work is not just the stepwise reduction in pore size, but the fact that each stage creates the working conditions for the next. If any stage is missing, the money spent on the subsequent stage will be wasted – not immediately, but gradually over three months, in the form of membrane fouling, soaring chemical consumption, and substandard permeate quality.
Stage 1: Pretreatment – the least glamorous, yet the most worthy of investment
Bar screens, equalization tanks, dissolved air flotation (DAF), coagulation‑sedimentation – these units may sound technologically mundane, but they offer the highest return on investment in the entire chain.
Bar screens remove fibers, plastic fragments, and larger particulates, with one sole purpose: to prevent physical damage to downstream pumps and membranes.
What is truly underestimated is the equalization tank. Industrial wastewater exhibits much more drastic quality fluctuations than municipal sewage: when a dyeing production line changes batches, COD can jump from 800 to 3000 mg/L within an hour; during floor‑washing periods in a food plant, instantaneous flow can triple the average. Without an equalization tank of sufficient residence time to equalize water quality and flow, the downstream biological system will be repeatedly shocked, and each recovery of the microbial community takes three to five days.
DAF and coagulation‑sedimentation address a different set of issues: oils, colloids, and suspended solids. Oily wastewater, if not reduced to below 20 mg/L at this stage, will form an irreversible fouling layer on the membrane surface. In one industrial park's project that achieves full water reuse and zero liquid discharge, the first section of the main treatment train is "bar screen → equalization tank → DAF → coagulation‑sedimentation" – four steps all devoted to pretreatment, which allows the subsequent A/O‑MBR to produce stable effluent.
Here is a counter‑intuitive conclusion: extra money spent on pretreatment is typically saved back three to five times over in reduced membrane replacement costs and downtime losses. The most expensive lessons in the industry almost all stem from "cutting corners on pretreatment" – yet this is precisely the section where cost‑cutting is least advisable in a reclaimed water treatment process chain.
Stage 2: Biological treatment – letting microorganisms handle the dissolved matter
Pretreatment deals with "visible dirt"; the biological stage handles dissolved organic matter and nitrogen in the water.
Conventional A/O, SBR, and contact oxidation processes essentially create a suitable environment for microorganisms to consume COD and ammonia nitrogen. The effluent from this stage typically achieves a COD of 50–100 mg/L, followed by clarification in a secondary clarifier.
Membrane bioreactor (MBR) takes a different approach: it uses membrane modules with pores around 0.1 μm to directly replace the secondary clarifier. The advantages are clear – effluent turbidity can be stably kept below 1 NTU, suspended solids are nearly zero, the mixed liquor suspended solids concentration can be two to three times that of conventional processes, and footprint can be reduced by about 60%. In retrofit projects with tight space constraints, MBR is often the only viable option.
It should be made clear that MBR solves the solid‑liquid separation within the biological stage; it does not remove salts. Clear effluent does not mean it can be directly reused in processes that have strict requirements for chloride ions. We have encountered clients who, seeing MBR effluent "as clear as bottled water," assumed that downstream deep treatment was unnecessary – only to find that the concentration factor in the cooling water system could not be increased, leading to scaling and corrosion problems.
Stage 3: Ultrafiltration – the bodyguard of reverse osmosis
Industrial ultrafiltration systems play a role in this chain that can be summed up in one sentence: they keep the downstream RO alive longer. RO membranes have rather stringent feed water requirements, primarily measured by two indices: turbidity and the Silt Density Index (SDI). Engineering practice generally requires feed SDI below 3 (stricter designs call for below 2.5) and turbidity below 0.2 NTU. Ultrafiltration membranes, with pore sizes between 0.01 and 0.1 μm, effectively block colloids, bacteria, macromolecular organics, and fine suspended solids at this gate.
Beyond protecting RO, ultrafiltration itself is often used as a terminal treatment unit. In many reclaimed water treatment projects, the reuse points are only cooling tower make‑up, workshop flushing, or landscaping, with no strict requirement for dissolved salts – in such cases, "MBR + ultrafiltration" is already sufficient, and RO is unnecessary. Ultrafiltration systems typically operate at 0.1–0.3 MPa, with energy consumption much lower than high‑pressure RO, and can achieve a recovery rate of over 95%.
Membrane material selection is also worth attention. PVDF (polyvinylidene fluoride) offers good chemical stability and mechanical strength, making it the mainstream choice for industrial wastewater; PES (polyethersulfone) has higher flux and better anti‑fouling properties, suitable for relatively clean water; Ceramic membranes are heat‑resistant and tolerant to strong acids and alkalis, but their cost is usually several times that of polymeric membranes, and they are only cost‑effective under special conditions.
Stage 4: Reverse osmosis – and the concentrate it cannot shake off
RO becomes a must‑have only when the reuse water has clear requirements for TDS, chloride ions, or conductivity. A single‑stage RO can reduce TDS from 800–1500 mg/L to 50–150 mg/L, with a salt rejection rate typically above 99%; double‑pass RO or RO with EDI can achieve conductivity below 1 μS/cm, meeting the demands for boiler feedwater and high‑purity water.
But RO has an unavoidable cost: concentrate. At 75% recovery, the concentrate volume is one‑quarter of the feed, with salt concentration four times that of the feed; at 90% recovery, the concentrate is only one‑tenth, with salt concentration ten times that of the feed. This stream must have a clear destination. Failing to consider the concentrate disposal route at the design stage is the most common – and most fatal – oversight in reuse projects; many systems are forced to run at reduced capacity after commissioning precisely because of this.
Final stage: Disinfection – and a detail often forgotten
In recirculating reuse systems, chlorine gas disinfection cannot be used casually. Chlorination generates disinfection by‑products such as trihalomethanes and haloacetic acids in the circulating water, and these substances will continuously accumulate in the closed loop. Reuse systems under the EU Regulation 2020/741 framework commonly switch to chlorine dioxide or ozone, combined with ultraviolet (UV) disinfection. UV has an additional benefit: it is equally effective against chlorine‑resistant pathogens like Cryptosporidium, and produces no by‑products.
The detail often forgotten is – the piping. Reclaimed water has different pH, alkalinity, and ionic composition compared to fresh water; if the existing carbon steel piping is used for the reclamation network, the corrosion rate may rise significantly. Conducting a pipe material compatibility assessment before commissioning is far cheaper than replacing pipes afterwards.
The process is not about stacking equipment
Putting all the above together, a complete water reuse treatment system typically looks like this:
bar screen → equalization tank → DAF/coagulation‑sedimentation → biological (A/O or MBR) → ultrafiltration → (RO) → disinfection → reuse point.
But what truly determines the success or failure of a project is not this sequence itself, but whether the parameters of each stage match the specific water you are dealing with. For the same capacity of 5,000 m³/day, an influent COD of 500 versus 5,000 will result in two entirely different engineering solutions; whether the reuse point is a cooling tower or a boiler may triple the investment.
That is why, when asked "what is your standard process?", Taihe Environmental Protection usually answers: give us a week of continuous influent data, and then we will talk about the process. Water reuse treatment technologies are never a fixed menu, but a toolbox of modular combinations – placing each stage where it belongs is more important than stacking the most advanced units.