Start from the standard, then choose the process
The usual order of selection is population, then flow, then process, but the process is not free to choose once the discharge standard is fixed. The standard sets the concentrations the plant has to meet on a daily average, and the concentration sets the treatment steps that can reach it. On GB 18918-2002 the grades run from Grade 1A, the strictest of the grades published on the standards chart, through Grade 1B to Grade 2.
Reading the limits, grade by grade
Grade 1A limits are COD 50 mg/L, BOD₅ 10 mg/L, suspended solids 10 mg/L, ammonia-N 5 mg/L, total nitrogen 15 mg/L and total phosphorus 0.5 mg/L. Grade 1B relaxes those to COD 60 mg/L, BOD₅ 20 mg/L, suspended solids 20 mg/L, ammonia-N 8 mg/L, total nitrogen 20 mg/L and total phosphorus 1 mg/L. Grade 2 goes further: COD 100 mg/L, BOD₅ 30 mg/L, suspended solids 30 mg/L, ammonia-N 15 mg/L and total phosphorus 3 mg/L, with no total nitrogen limit in the table.
The gaps between the grades are not even. Moving from Grade 2 to Grade 1B roughly halves the organics limit. Moving from Grade 1B to Grade 1A halves the suspended solids and phosphorus limits again, and those two are the numbers that decide whether a conventional clarifier is enough.
It helps to read the numbers as treatment duties rather than as a score. A suspended solids limit of 10 mg/L sits close to what a well-run clarifier can hold, so the Grade 1A figure is usually met with a filter or a membrane rather than by settling alone. A total phosphorus limit of 0.5 mg/L needs a biological phosphorus removal step or chemical dosing, because ordinary biological treatment does not reach it reliably. Grade 1B, at 1 mg/L of phosphorus and 20 mg/L of suspended solids, is the more forgiving target.
The ammonia bracket rule
Ammonia-N is the parameter that catches people out, because the applicable limit depends on temperature. Grade 1A reads 5 mg/L, with 8 mg/L in brackets. The value outside the brackets applies when the water temperature is above 12 °C, and the bracketed value applies at or below 12 °C. Grade 1B is written 8 and 15, and Grade 2 as 15 and 30, on the same rule.
A design compared against the wrong half of the bracket can look compliant in summer and non-compliant in winter, or the reverse. The bracket rule should be read before any comparison, not after.
From grade to process train
Each grade maps to a process combination that normally delivers it. Grade 2 and Grade 1B are served by an A/O or A²/O train with a secondary clarifier and disinfection; where only organics are controlled, the biology can fall back to biological contact oxidation. Grade 1A needs more: an A²/O or MBBR train with a secondary clarifier followed by advanced treatment — coagulation sedimentation, a deep-bed filter and disinfection — or an MBR directly.
The MBR route is worth understanding because it trades one cost for another. It omits the secondary clarifier and takes less space, but energy use and membrane replacement cost more; at small flows it is often the better value. That is why a packaged MBR is a common answer where a Grade 1A limit meets a small site. The process combinations are common engineering practice rather than a mandatory requirement of the standard, so they are a starting point for the design, not the rule itself.
Extended aeration is the other common answer, and it is the Middle East convention. The design guide for that region describes it as an aeration time of 18–36 h, a mixed liquor suspended solids of 3,000–6,000 mg/L and a sludge age of 10–30 days. It takes more tank volume than an MBR, but it has no membrane to replace, which suits a site where spares and specialist maintenance are hard to reach.
Reuse: the standard is not the only limit
Where the treated water is to be reused, meeting the discharge standard is not the end of the question. A reuse scheme is governed by the limits for the application it serves, and a plant that discharges compliantly may still need further treatment for irrigation, landscaping or process use. MBR with disinfection covers most reuse duties, with MBR and reverse osmosis where a high degree of desalination is required. The water-quality limits for the reuse application come from the reuse standards rather than from the discharge standard.
Reuse water quality is governed separately, and the chart points to two standards for it: GB/T 18920-2020 and GB 50335-2016. A reuse target should be fixed during sizing, because the treatment step that reaches it is added before the plant is built rather than after.
Exporting into Europe
Projects outside the national standards work to a local permit, and in Europe the product dimension is covered separately by EN 12566-3. A plant entering the UK or EU market normally carries an EN 12566-3 type test alongside the local discharge permit, and in the UK the discharge is also subject to the permit and to the drainage-field conditions. The two requirements are not interchangeable: the type test describes the product, and the permit governs the discharge at the site.
The destination country sets the limits, not the supplier
A plant exported to another country is governed by that country's limits, and those limits are set by its environmental authority and written into a permit for the individual site. Nothing in the equipment supply changes that, which is why the first task on an export project is not a process choice but a document search: find the authority that issues the discharge permit, and obtain the regulation that authority publishes rather than a limit table passed on by a supplier.
Two layers usually exist. A national instrument carries the general limits for the sector, and a second authority can tighten them for the particular site — a basin or catchment authority, a province, or a municipality. Where the receiving water is classified as sensitive, or where the discharge goes to a protected catchment, the permit will be stricter than the national figure. The permit also carries the practical detail a compliance test follows: the parameters, the sampling points and the averaging period.
- The Philippines: the general effluent standards are published by the Department of Environment and Natural Resources (DENR) through its Environmental Management Bureau, as a DENR Administrative Order. The permit for a site is issued by the DENR regional office, and a plant in the Laguna de Bay basin also falls under the basin authority.
- Indonesia: national sector standards are set at ministry level by the Ministry of Environment and Forestry (KLHK) in a ministerial regulation, and provincial or district regulations can set stricter values for the same sector — that local layer is where an imported design is most often caught out.
- Saudi Arabia: the environmental regulator is the National Center for Environmental Compliance (NCEC), and earlier permits in the market were written against the Presidency of Meteorology and Environment (PME) environmental protection standards, which remain the reference many projects carry. The limits are stated per discharge route, so the route has to be fixed before the limit is read.
- In all three cases the design is fixed against the permit for that site, and the national instrument is only the fallback where the permit is silent. A permit obtained late is a design change; a permit obtained during sizing is an input.
What a supplier can do is ask the right questions early. Which authority issues the permit, and has the current text of its regulation been obtained from the authority itself? What is the receiving water classified as? Does a second authority set a tighter value? The limit tables of other countries are not reproduced on this page, because they change and because a design fixed against a stale copy is worse than one fixed against nothing.