Effluent Quality — BOD, Suspended Solids and Coliform Count
The three tests that decide whether treated sewage is clean enough to discharge, what each one actually measures, and the numbers that go with them.
Key Principles at a Glance 7 points
- Three parameters describe effluent quality: BOD, which measures the polluting power left in it; suspended solids, which measure appearance and silting; and coliform count, which measures health risk.
- BOD is determined by incubating a well-oxygenated sample at 20 °C and measuring the oxygen absorbed over five days; raw sewage runs 300 to 600 mg per litre against an IMO recommendation of under 50 after treatment.
- Suspended solids and BOD rise together, because the same failure — solids escaping the settling chamber — causes both, and that is what makes the test diagnostic.
- Suspended solids are measured by filtering through a pre-weighed pad, drying and re-weighing, and are reported in parts per million or mg per litre.
- The e-coliform count is used as an indicator rather than a direct hazard measurement, and it is the test that tells you whether the chlorinator is working.
- The limits differ by source: IMO recommends under 250 faecal coliforms per 100 ml after treatment, the US and Canada standard for restricted waters allows 1000 per 100 ml, and 200 per 100 ml is commonly quoted.
- Each person contributes around 125 billion coliform organisms in winter and 400 billion in summer, so a plant serving thirty people handles a load measured in trillions per day.
1. The three parameters
The standard that applies is the one set by the local or national authority for the waters the ship is in. The figures in this chapter are the commonly quoted limits; the authority in force governs.
To discharge sewage in territorial waters the effluent quality may have to be within standards laid down by the local or national authorities. These are usually based on one or more of three factors: the bio-chemical oxygen demand, the suspended solids content, and the e-coliform count.
Each measures something different, and the three together describe the effluent fairly completely:
- BOD measures how much oxygen the effluent will take out of the water it is discharged into. It is a measure of the remaining polluting power.
- Suspended solids measures what is physically floating in it. It is a measure of appearance and of silting.
- Coliform count measures how much human waste is present. It is a measure of health risk.
2. Bio-chemical oxygen demand
BOD is determined by incubating, at 20 °C, a well-oxygenated sample of effluent. The amount of oxygen absorbed over a five-day period is measured.
The test evaluates the effectiveness of treatment, because it measures the total oxygen taken up as final and complete breakdown of organic matter by aerobic bacteria occurs. The quantity of oxygen used equates to the amount of further breakdown required.
Put plainly: if the effluent still has a high oxygen demand, then whatever is in it will still be consuming oxygen in the sea, and the treatment has not finished the job. A low BOD means the organic matter has already been broken down on board, where it can be managed, rather than in the water.
The purpose in one line: when we discharge the water out, aerobic bacteria should not go alive — that is, the effluent should not carry a load that will strip oxygen from the receiving water.
The numbers
| Value | |
|---|---|
| BOD of raw sewage | 300 to 600 mg per litre |
| IMO recommendation after treatment | Less than 50 mg per litre |
| Expression of the result | The amount of oxygen taken by a 1 litre sample incubated at 20 °C |
The comparison is the point. Raw sewage at 300 to 600 against a treated limit of under 50 is roughly a tenfold reduction, and that reduction is what the aeration chamber is doing.
3. Suspended solids
Suspended solids are unsightly, and over a period of time they give rise to silting problems. They are usually a sign of a malfunctioning plant, and when very high they are accompanied by a high BOD.
That last sentence is the diagnostic value of the test. Suspended solids and BOD rise together, because the same failure — solids escaping the settling chamber — causes both. A high suspended solids reading points at the settling chamber, the sludge return, or the surface skimmer, rather than at the aeration chamber.
How it is measured
Filter a sample through a pre-weighed pad, then dry it and re-weigh it. The increase in weight is the suspended solids. The results are in parts per million, or milligrams of suspended solids per litre.
Older descriptions of the same test use an asbestos mat filter element rather than a pre-weighed pad, with the solids collected, dried and weighed. The principle is identical; only the filter medium has changed.
4. Coliform count
The e-coliform is a family of bacteria which live in the human intestine. They are quantified by a laboratory test, and the result indicates the amount of human waste present in the sample. The result is called the e-coli count, and it is expressed per 100 ml.
Why it matters
They are not normally harmful, except when they contain pathogenic colonies which can cause dysentery, typhoid, para-typhoid and gastro-enteritis.
Apart from these bacteria there are also viruses of the type responsible for poliomyelitis and infectious hepatitis.
The coliform count is therefore used as an indicator rather than as a direct hazard measurement. It is easy to culture, and a high count means human waste is present — which means the pathogens that accompany human waste may also be present.
The effectiveness of disinfection is checked by a coliform count on a sample of effluent. This is the test that tells you whether the chlorinator is working.
The limits differ by source
This is the part that catches people out, because there is no single number. Quote the right one for the question asked.
| Source | Limit |
|---|---|
| IMO recommendation | Faecal coliform less than 250 per 100 ml after treatment |
| US and Canada standard for restricted waters | Coliform not to exceed 1000 per 100 ml |
| Commonly quoted figure | Coliform count 200 per 100 ml maximum |
The American Type I and Type II standards also carry coliform limits, but they are complete effluent specifications rather than coliform figures on their own — Type II sets a suspended solids limit as well. They belong with the rest of the American definitions, in The Discharge Regulations, section 3.
The reason for the spread above is that these figures come from different instruments — an IMO recommendation and regional standards — and they apply in different waters. The safest course is to give the figure for the authority named, and to say which authority it comes from.
Incubation and scale
One test takes 48 hours at 35 °C; another produces a colony in 24 hours at 35 °C. Either way, the result is not available for a day or more — which is why the on-board monitor works optically and instantly, and the coliform test is a laboratory confirmation rather than a control.
For scale: each person contributes around 125 billion coliform organisms in winter and 400 billion in summer. A plant serving thirty people is therefore dealing with a coliform load measured in trillions per day, and the disinfection stage is what brings it down to hundreds.