A specification is what makes a quotation a price
A supplier asked for a price on a packaged sewage treatment plant without a specification can only answer with an estimate, because several independent numbers are still free. Design flow, influent strength, discharge target, tank size and shipping envelope each move the equipment that is offered, and until they are fixed the price is a range rather than a figure.
The order below is deliberate, because each figure constrains the next, and taking them in the wrong order leads to a plant chosen before its duty is known.
1. Design flow and the peak factor
The first figure is the design flow, and it comes from the people or the process the plant serves. For domestic sewage, national practice converts water use into sewage at a fixed rule: the combined domestic sewage allowance is 90% of the local water-use allowance, banded from 50 to 320 L per person per day depending on the water-resource zone and city size. State the served population and the allowance band used; where the flow comes from an industrial process or a water meter, state that source and the measured flow instead.
Give the figure as an average daily flow, because the peak is a separate input. The dry-weather variation factor Kz scales the average up to the peak, and national practice gives values falling from 2.7 at 5 L/s to 1.5 at 1,000 L/s and above (GB 50014-2021). One caution belongs in the specification: the 2021 edition raised these factors against the superseded 2006 set, so older material may still be applying values from 2.3 down to 1.3.
2. Design influent quality
Hydraulic flow alone does not fix the plant, because the biology is sized on what the water carries. Municipal sewage concentrations are published as industry statistics rather than standard values: a national survey of 127 treatment plants gives mean COD of 220 mg/L, BOD₅ of 81.6 mg/L and suspended solids of 148.5 mg/L, with the 80th-percentile column and a common design range alongside. The influent-quality chart sets out all three columns.
Where the site has measured influent data, use it and say so. Where it does not, the common design range is the planning default, and the range should be named rather than a single figure, because a plant specified on the mean of a survey is sized for half its days. Sewage with a high industrial fraction needs its own sampling under GB/T 31962-2015.
3. Discharge standard and target grade
The target is a permit question before it is an equipment question, and the grade should be written into the specification with its parameter values. On GB 18918-2002, Grade 1A is 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 to COD 100 mg/L, BOD₅ 30 mg/L, suspended solids 30 mg/L, ammonia-N 15 mg/L and total phosphorus 3 mg/L.
Two details decide the process. Ammonia-N carries a bracketed value, and the applicable number depends on water temperature: the value outside the brackets applies above 12 °C and the bracketed value at or below 12 °C, so a plant specified against the wrong half of that bracket can comply in summer and fail in winter. Where the treated water is to be reused rather than discharged, the reuse limits apply as well. The effluent-standards chart maps each grade to the process combination that normally delivers it.
4. Tank volume, residence time and box dimensions
With flow and load fixed, the tank volume follows from the residence time the process needs, set per stage. National practice gives the anaerobic tank 1–2 h and the anoxic tank 0.5–3.0 h, with a solids retention time of 3.5–7.0 days where phosphorus removal is the main objective. State which figure the design is held to, and whether the tank is sized on the average or the peak flow.
The volume then becomes a physical envelope with a shipping consequence. A single containerized unit covers a design flow of 0.5–15 m³/h, and the internal width of a standard dry container is 2.352 m, so any unit wider than that cannot travel in one. Give the maximum single-piece width and height the site can receive, because that pair decides the shipping route.
5. Site conditions the equipment has to meet
Four site inputs change what can be offered. Power: the available supply, its voltage and its stability, because aeration and membrane operation are continuous loads. Water temperature: the winter minimum matters more than the summer value, because the ammonia limit depends on it. Operating mode: whether the plant runs attended or unattended, which decides how much instrumentation is worth fitting. Footprint and access matter too, because they constrain the box that can be delivered.
6. The list to hand over before a price is fixed
- Served population and per-capita allowance, or the measured process flow.
- Average daily flow, and the peak factor Kz with its source.
- Design influent quality, from measurement where it exists and the common design range where it does not.
- Discharge standard and grade, with the parameter values and the temperature rule for ammonia.
- Residence time and solids retention time, and whether the tank is sized on average or peak flow.
- Maximum single-piece width and height, plus available power and winter water temperature.
Those inputs are what a supplier needs to replace an estimate with a price, and handed over together they make the offers comparable.
7. What the specification does not decide
Fixing these inputs does not fix the process. A plant can meet the same grade with an A²/O train and a clarifier, with a membrane, or with a biofilm process, and the choice between them belongs to the commercial conversation. What the specification does is remove the freedom to guess: once the flow, the load, the target and the envelope are written down, the price that comes back is a price for the plant described.