Sewage Flows and Loads: Per Capita Design Values, Non-Domestic Sources, Phosphorus Apportionment and Tankered Waste

The domestic per capita basis

UK practice, codified in the British Water code of practice on flows and loads used by the environmental regulators for small works, takes a standard resident as:

That 150 L/head/d is the sewage contribution, not the water supplied; measured per capita consumption in metered UK households is 120 to 140 L/head/d, and a proportion does not reach the sewer. In the code, dwellings are sized by occupancy rather than by headcount, with a minimum population per house that rises with bedrooms, so that a development of 20 three-bedroom houses is designed for its potential occupancy rather than its census count.

Loads matter more than flow for process sizing: a 2,000 PE works treats 120 kg BOD/d and 16 kg NH₄-N/d, and those two numbers, with the permit, size the biological stage.

Non-domestic sources

The code gives flow and load per person or per activity for the common non-domestic sources. Flow and BOD per person vary by a factor of ten depending on whether meals are cooked and how long people stay:

Three points of practice. Count every source on the site and its peak occupancy, not its average. Ask about the activity, because a "factory" with a canteen and showers is a different load from one without. And treat any process effluent (dairy, brewery, laundry, vehicle wash) separately with its own characterisation, since the per-person figures cover sanitary flow only. Strength, the ratio of BOD to flow, also matters: a conference centre at 150 g BOD in 350 L is 430 mg/L, much stronger than domestic sewage at 400 mg/L and far stronger than an office at 250 mg/L; strength drives the aeration and settlement design as much as the mass does.

Infiltration, DWF and peaks

Dry weather flow is the sum of population flow, infiltration and trade effluent, DWF = PG + I + E, as described in the flow measurement guide. Infiltration is the term most often omitted and most often wrong: on an old sewer network it can equal the population flow, and it is measured, not assumed, from minimum night flow at the works in dry weather. A new, tested system may be designed at 10 to 20 percent of PG; an existing catchment must be gauged.

Peak flows for a separate foul system without storm input are set by the diurnal pattern. Small catchments peak sharply (a factor of 4 to 6 on the daily mean for a few hundred people, in the morning), large ones flatten to 1.5 to 2. Flow to full treatment on a combined system is fixed by the permit formula, 3PG + I + 3E. Load peaks lag flow peaks and are smaller, since concentration falls as flow rises; a load factor of 1.5 to 2 on the daily mean is normal for the morning peak at a small works, and settling, equalisation or process selection has to absorb it. A works receiving a lunchtime hotel load or a school discharge will have a second peak that the standard diurnal curve does not show.

Where domestic phosphorus comes from

Phosphorus loads matter because most inland permits now limit it, and because the source split tells you what can and cannot be changed upstream. UK source apportionment of the domestic load gives about 40 percent from the natural diet, 29 percent from phosphate food additives (processed meats, baked goods, drinks), 14 percent from laundry products including phosphonates, 9 percent from automatic dishwasher detergents, 6 percent from the orthophosphate dosed to drinking water to suppress lead, and about 1 percent each from food waste disposers and personal care products. Dietary intake alone is about 1.3 g P/person/d from natural foods and roughly 0.65 g more from additives and supplements; dishwasher detergent contributed about 0.17 g P/person/d and laundry products about 0.15 g at the time of the apportionment.

Design implications: the laundry contribution has been falling since the EU and UK phosphate limits on laundry detergents took effect, and the dishwasher figure may follow, but the dietary and additive share, more than two thirds of the load, is not controllable at source. A design value of 1.8 to 2.5 g P/person/d, confirmed by sampling at an existing works, remains appropriate; where lead-pipe phosphate dosing is practised in the supply, the upper end applies. Removal technologies are covered in the chemical phosphorus removal guide and the biological nutrient removal guide.

Tankered cesspool and septic tank waste

Many works accept tankered domestic waste from cesspools and septic tanks, and it is not sewage. Stored for weeks, it is septic, concentrated by low water use, and carries settled solids. Monitoring at a works receiving tankered cesspool waste found the tankered material had a median BOD₅ around 3,800 mg/L against 240 mg/L in the sewer inflow, with COD and total nitrogen elevated in similar proportion: roughly 16 times the strength of the sewage. The same study found that keeping the tankered share to about 5 percent of the daily flow kept the works' unit loads at their design values, and that the damage came less from the daily share than from its irregularity: a week's deliveries discharged on one morning.

A reception facility sized and operated for the load, not the volume, is the design response: a screened, sampled reception tank with capacity for at least a day's deliveries, metered bleed into the inlet at a rate set by the works' load capacity (for a 2,000 PE works at 120 kg BOD/d, a 5 percent load allowance is 6 kg BOD/d, which at 3,800 mg/L is only 1.6 m³/d of tankered waste), a waste acceptance procedure that records origin and rejects trade waste disguised as domestic, and odour control at the reception point because the material is already septic, as described in the septicity guide. Where tankered volumes are commercially important, the works must be designed with the extra load from the outset; an afterthought reception point is the most common cause of a small works failing its permit on Mondays.

Building the flows and loads table

The deliverable is a single table, per source and in total, of average daily flow, infiltration, trade effluent, DWF, peak flow, flow to full treatment where relevant, and the daily mass and concentration of BOD, COD, suspended solids, ammonia, total nitrogen and phosphorus, with the seasonal and weekly variation stated. It should name its source for every figure, whether code of practice, measurement or assumption, and show the sensitivity of the plant size to the two or three assumptions that matter most, usually occupancy, infiltration and any non-domestic load. This table is the contract between the client, the designer and the regulator, and when the treatment plant is sized against it, it is the document the permit application rests on. For an existing works the table is replaced by a monitoring campaign, which the characterisation guide describes.

How to build a flows and loads basis for a sewage treatment design

1. List every source. Dwellings by bedroom-based occupancy, every non-domestic activity with its peak occupancy, every process effluent separately.
2. Apply per capita and per activity values. Code of practice figures for sanitary flow and load; measured data for process effluents; design phosphorus from sampling or 1.8 to 2.5 g per person per day.
3. Add infiltration and trade effluent. Measured night flow on an existing network; 10 to 20 percent of population flow for a new tested system.
4. Derive DWF, peaks and flow to full treatment. DWF = PG + I + E; peak factor from catchment size and any non-diurnal sources; FFT from the permit formula where storm flows enter.
5. Allow for tankered waste. Set the acceptable daily load (about 5 percent of design load), size the reception tank and bleed rate, and write the acceptance procedure.
6. State sources and sensitivities. Reference every figure and show how plant size moves with the key assumptions before the table is frozen.