High-level biological purification of coking plant waste water
The buffer reservoir
In the coking plant, coals are converted into coke. During that process, organically polluted waste water is formed. The pollutants are broken down microbiologically in oxygen injection basins, after which the nitrates from the effluent are converted into nitrogen. By balanced and gradual loading of the sludge basins and by strictly controlling their oxygen concentration, sludge load, temperature and acidity, 90% of the COD load (which stands for Chemical Oxygen Demand and is used to measure the oxidising components in water) and 75% of the nitrogen load are removed from the effluent. This facility is in fact one of the most efficient water treatment facilities in European industry.
The biological water purification facility of the coking plant consists of four large components:
- the buffer reservoir
- the sedimentation of tar
- the oxidation basins
- the final sedimentation
The buffer reservoir fulfils a threefold task:
- homogenise incoming water;
- catch any possible flow fluctuations upstream, so that a constant influent flow can be drained off to the active sludge treatment unit;
- remove all colloidal particles from the effluent through coagulation (form floc) and flocculation (precipitate floc) before the biological treatment.
To stimulate the sedimentation of these particles, the reservoir has an internal funnel and an overflow pipe. The space around the funnel is filled non-stop with hot influent through the overflow pipe for the optimum sedimentation in the funnel. A pump with an added flow control valve, continuously sends a constant amount of influent to the tar sedimentation tank through a heat exchanger. The heat exchanger keeps the temperature of the water stable to avoid heavy temperature fluctuations caused by the season, the weather or day- or nighttime during its biological treatment.
The tar sedimentation basin
Oils, tar particles and other suspended substances are separated in the tar sedimentation basin. The sedimentary sludge is continuously transported by a scraping chain to a funnel-shaped pit, from which it can discontinuously be drained off to a sludge thickener. The separated oil fractions are discontinuously removed with a skimmer very close to the overflow.
The effluent of the tar sedimentation basin is divided into four oxidation basins. The first oxidation basin is used anaerobically, in the absence of oxygen, during 50 minutes an hour in order to convert the formed nitrates from the final basin into N2. In the final three oxygen injection basins, the aerobical biodegradation of phenols, thiocynates, cyanides, thiosulphates, polythionates and ammonial nitrogen is effected. Because the waste water contains insufficient phosphorus, phosphoric acid is dosed and discontinuously added as a nutrient. The acidity in the basins is kept optimal by adding sodium hydroxide.
Bacteria at work
The micro-organisms that purify the waste water biologically, are bacteria that are waiting in our water purification basins ready to attack the pollutants. It is amazing how varied and subtle the micro-organisms cooperate. The cycle is complete and nothing goes to waste.
The scheme above shows that the micro-organisms of the nitrobacter familiy need the nitrosomonas family which in turn is dependent on the thiobacillus family, which in turn needs the pseudomonas family which, in conclusion, cannot do without the nitrobacter family.
Cooperation is vital for animals and plants to survive. Being aware of that has a great impact on the common image of nature. Lewis Thomas, physician and author, wrote in 1980 that the general outlook of nature is an important issue that has to be studied. A century ago, everyone was sure that in nature, there is a constant struggle for survival. Evolution would have been the result of a direct struggle between competing species, in which the most persistent survive (“survival of the fittest”!). But now, it seems that this is a misrepresentation. The tendency to cooperate may very well be the oldest, strongest and most fundamental power in nature. There are no rogue, independent living souls. Every living being is dependent on others.” The biological water treatment facility at ArcelorMittal Gent clearly exemplifies that principle.
Final sedimentation basin
In the final sedimentation basin, micro-organisms are separated as much as possible from the biologically purified water. The sludge settles down on the bottom of the final sedimentation basin and is transported by a scraping chain to two funnel-shaped pits from which 76 m3/h is continuously led back to the oxygen injection basins. A small amount of drainage sludge (2 m3/h) discontinuously goes to the sludge thickener where it is mixed with the sludge from the tar sedimentation basin and the buffer reservoir. In the sludge thickener, the sludge is thickened until it has a dry dust content of 7%. After the sludge is dried, it is transported to the coal supply unit of the coke oven in order to process the sludge together with the coals in the coke oven. As a result, the waste flow is a closed circuit causing no nuisance or harm to the environment.