Engineering Hub: get this sized for your plant
Describe your goal, effluent limit or sludge volume and receive a technology shortlist plus a sizing proposal – by email, no phone call required.
From discharge permit to reuse permit: a different compliance logic
Treating effluent to discharge standards and treating it to reuse standards are fundamentally different tasks. A discharge permit tolerates occasional exceedances that average out over time. A reuse permit — particularly under the EU Water Reuse Regulation (EU 2020/741) — requires continuous compliance: every cubic metre delivered to the irrigation system must meet the quality threshold, not just the monthly average.
For Class A reuse, the two parameters that drive the monitoring design are TSS and turbidity. This article explains what the regulation requires, how filtration delivers it, and how to build a monitoring chain that documents compliance without gaps.
EU Water Reuse Regulation — Class A Irrigation
- TSS limit
- ≤ 10 mg/l
- Turbidity limit
- ≤ 5 NTU
- E. coli limit
- ≤ 10 CFU/100 ml (after disinfection)
- Legionella limit
- ≤ 1000 CFU/l (for spray irrigation)
- Monitoring frequency
- Continuous for turbidity; daily for TSS
What Class A actually requires from the filtration step
The regulation sets the quality at the point of delivery to the user, not at the outlet of the treatment plant. This means the filtration and disinfection steps must together achieve the Class A limits, and the monitoring must cover the full chain.
Turbidity is the key surrogate parameter. At ≤ 5 NTU, turbidity correlates reliably with TSS ≤ 10 mg/l and — critically — with the UV transmittance needed for effective disinfection. A turbidity spike above 5 NTU does not just breach the TSS limit; it also reduces UV dose delivery and can push E. coli counts above the Class A threshold.
This is why the regulation requires continuous turbidity monitoring rather than grab sampling: a 30-minute turbidity exceedance during a storm event is invisible in a daily composite sample but can contaminate an entire irrigation cycle.
≤ 5 NTU
Turbidity limit for Class A reuse (EU 2020/741)
≤ 10 mg/l
TSS limit for Class A reuse
10 CFU/100 ml
E. coli limit after UV disinfection at Class A
How pile cloth media disc filters deliver Class A TSS and turbidity
Pile cloth media disc filters achieve effluent turbidity of 1–3 NTU and TSS of 2–5 mg/l under normal secondary effluent conditions — well within Class A limits. The key design parameters are:
- Hydraulic surface loading rate: 5–10 m/h for secondary effluent with TSS 10–30 mg/l. Higher loading rates increase effluent turbidity.
- Cloth pile depth: longer pile fibres capture finer particles but require more frequent backwash. For reuse applications, medium-pile cloth (3–5 mm) balances filtration efficiency with backwash water consumption.
- Backwash water quality: backwash water must be returned to the head of the plant, not to the reuse stream. Backwash volume is typically 1–3 % of throughput.
Turbidity Spike Management
Install an automatic diversion valve downstream of the filter. When online turbidity exceeds 4 NTU (set point below the 5 NTU limit), the valve diverts flow back to the treatment plant rather than to the UV disinfection unit. This prevents a filtration upset from propagating to the reuse stream.
Building the continuous monitoring chain
A compliant Class A monitoring chain has four elements:
Class A Monitoring Chain
- Online turbidimeter immediately downstream of the filter (before UV)
- Online turbidimeter immediately downstream of UV disinfection (at delivery point)
- Automatic diversion valve triggered by turbidity set point
- Daily TSS grab sample from the delivery point (laboratory analysis)
- Weekly E. coli sample from the delivery point
- Monthly Legionella sample if spray irrigation is used
- Data logger with tamper-evident records for regulatory audit
The online turbidimeter before UV serves two purposes: it triggers the diversion valve and it provides the UV system with a real-time transmittance proxy to adjust UV dose. The turbidimeter after UV confirms that disinfection has not introduced turbidity (e.g., from lamp sleeve fouling).
Sizing the filtration step for reuse
- 1Characterise the secondary effluent
- 2Measure TSS, turbidity, and particle size distribution over at least four weeks including storm events. Peak TSS during storm events often exceeds 30 mg/l and must be covered by the design.
- 3Select hydraulic loading rate
- 4For Class A reuse, design at 6–8 m/h to provide a safety margin below the 10 m/h upper limit. This allows the filter to handle moderate TSS spikes without exceeding 5 NTU.
- 5Size the filter area
- 6Divide peak flow rate by the design hydraulic loading rate. Add 20 % margin for one disc out of service during maintenance.
- 7Specify the diversion valve
- 8Size the valve for full peak flow. Set the turbidity trigger at 4 NTU to provide a 1 NTU buffer before the 5 NTU limit.
- 9Integrate with UV system
- 10Confirm that the UV system accepts a 4–20 mA turbidity signal for dose control. Specify minimum UV dose for Class A: 40 mJ/cm² at peak flow and maximum turbidity.
Does the EU Water Reuse Regulation apply to industrial sites that reuse their own treated effluent?+
The regulation applies to treated urban wastewater supplied for agricultural irrigation. Industrial sites reusing their own process water are generally subject to national industrial water reuse frameworks rather than EU 2020/741 directly. However, many national frameworks reference the EU regulation's quality classes, and Class A limits are increasingly used as the benchmark for any irrigation reuse regardless of the water source.
Can a sand filter meet Class A turbidity limits?+
A well-operated sand filter can achieve 5–10 NTU under normal conditions, but maintaining consistent performance below 5 NTU during storm events or secondary effluent quality fluctuations is difficult without coagulant dosing. Pile cloth media disc filters achieve 1–3 NTU without coagulant under typical secondary effluent conditions, providing a larger safety margin for Class A compliance.
How often must the online turbidimeter be calibrated?+
The EU regulation does not specify a calibration interval, but regulatory guidance and instrument manufacturer recommendations typically call for verification against a reference standard every 1–4 weeks and a full calibration every 3–6 months. Calibration records must be retained for audit. A two-point calibration (zero and span) using certified formazin standards is the accepted method.
Equipment we supply for this
Engineering Hub: get this sized for your plant
Describe your goal, effluent limit or sludge volume and receive a technology shortlist plus a sizing proposal – by email, no phone call required.