A small flow with a high duty
A hospital or a clinic produces a small flow with a heavy duty. The population is modest, but the water reaching the drain carries pathogens, pharmaceuticals and a nutrient load that a municipal plant of the same hydraulic size would not see. Sizing on the flow alone is the first mistake made on these sites, because the biological load and the disinfection duty are what set the equipment.
The flow is also not steady. A ward fills and empties with the working day, and the water arriving at the plant follows that pattern with a lag. A small population produces a sharper peak than a large one, so the plant is sized on the peak and on the equalization volume that smooths it.
What makes it different from municipal sewage
Four things separate hospital wastewater from the municipal sewage the usual sizing method is built around.
- Pathogens. The stream carries infectious material at a concentration a domestic sewer does not, which makes disinfection a duty rather than an option.
- Ammonia. The nitrogen arrives mainly as ammonia, and the ammonia load is high against the flow. The sizing method makes the same point from the other direction: where the population is not the driver, a hospital included, the flow comes from the water meter or the process brief rather than from a per-capita allowance.
- Intermittency. The flow arrives in a pattern set by the working day rather than spread across it, so the equalization volume does more work than at a municipal plant.
- Chemical residues. Iodinated contrast media, disinfectants and pharmaceutical residues enter the drain with the ordinary use of the facility, and they change what the biological stage sees.
The result is a plant that is hydraulically small and biologically heavy, which is the opposite of the case the per-capita sizing route is designed for.
Disinfection is not optional
Disinfection is the stage that separates a hospital plant from a domestic one, and the method is a site decision. Chlorination is the most common route because it is simple and leaves a residual that keeps working after the contact tank; its limits are the residual itself and the compounds it can form with organic matter. Ultraviolet leaves no residual and adds nothing to the water, but it needs clear water, because solids shade organisms from the light. Ozone is the most reactive and the most demanding to operate, and is usually reserved for a reuse duty.
The packaged plants described here carry disinfection as part of the train. The containerized unit lists chlorination, UV or ozone depending on the discharge or reuse target, and the membrane-based plant follows its membrane stage with final pathogen reduction. Where the treated water is reused for irrigation or flushing, the membrane and disinfection combination is the usual answer.
The nitrogen duty
Nitrogen decides how much tank the plant needs. Because the nitrogen arrives mainly as ammonia, the plant has to nitrify before it can remove it at all: ammonia is oxidised to nitrate in the aerobic zone, and only then can the nitrate be turned into nitrogen gas in an anoxic zone. A plant built for organic removal alone will not hold an ammonia limit.
The discharge limit adds a second constraint. On GB 18918-2002 ammonia-N is written with 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. Grade 1A reads 5 mg/L with 8 mg/L in brackets. A design compared against the wrong half can look compliant in summer and fail in winter, and a hospital is exactly where that happens, because the plant is often commissioned late in the year.
Nitrification also slows in cold water, so the aerobic volume needed to hold the ammonia limit in winter is larger than the summer case suggests.
What must not reach the drain
Not everything a hospital uses should reach the treatment plant, and the most effective step is taken before the drain rather than inside it. Iodinated contrast media, laboratory reagents, mercury, formalin and cytostatic drug residues belong in a separate collection route, because a biological plant cannot remove them and disinfection does not destroy them.
The practical form of this is source separation: collect these materials in a dedicated container in the department that uses them, and they never enter the plant. Where they do enter the drain, they pass through and appear in the effluent, which is a compliance problem rather than a treatment problem. Source control is a hospital procedure rather than a piece of equipment, and it belongs in the same plan as the treatment plant.
Sizing it
Hospital wastewater is sized on the load the facility produces rather than on a simple per-head figure. Two inputs serve that purpose: the number of inpatient beds, which set the base flow and the night-time load, and the outpatient and day-clinic activity, which adds the peak and the chemical load the beds do not produce. Where the site has a water meter and a drainage record, those replace the estimate.
The target grade then decides the process. A hospital discharge to a water body usually aims at the strictest grade the permit allows, and Grade 1A is the strictest published here: 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. That combination plus disinfection is normally met by a membrane-based packaged plant, which reaches the grade directly, or by an A²/O or MBBR train with advanced treatment after it.
At the small flows a clinic or a district hospital produces, the membrane route is often the better value: it omits the secondary clarifier, and the packaged range here covers 12–240 m³/day for a membrane plant and 12–360 m³/day for a containerized unit. A site near a city can also discharge to the municipal sewer and leave part of the duty to the municipal works.
The standard and the permit
The limit a hospital has to meet depends on where the water goes. A plant discharging on site, to a watercourse or to the ground, is governed by the discharge standard the local authority applies, which in the GB system is GB 18918-2002 with the grades above. A hospital discharging to a municipal sewer instead is governed by GB/T 31962-2015, the standard for wastewater discharged into a municipal sewer, and the municipal plant then carries part of the duty.
The choice between the two routes belongs to the project and the local permit. The permit fixes the target, and it should be read before the process is chosen, because the two routes lead to plants of different sizes. Where the permit is not yet issued, the sizing should state the grade it is designed against.