Eliminating purification sludge

The sludge from API oil separator bottoms is dealt widi directly by incineration in an outside facility. Amounts represent about 500 t/year at a concentration of 150 to 250 g l h The oil collected at the API surface with flotation sludge (20% of the total) in slop tanks comes to 2500 m -y , with humidity under 10%. It is reused in the refinery s industrial fuel network. 

---

Solvent extraction—purification of wet-process phosphoric acid is based on preferential extraction of H PO by an organic solvent vs the cationic impurities present in the acid. Because selectivity of acid over anionic impurities is usually not sufficient, precipitation or evaporation steps are included in the purification process for removal. Cmde wet-process acid is typically concentrated and clarified prior to extraction to remove post-precipitated sludge and improve partition of the acid into the solvent. Concentration also partially eliminates fluoride by evaporation of HF and/or SiF. Chemical precipitation of sulfate (as Ba or Ca salts), fluorosiUcates (as Na salt), and arsenic (as sulfides) may also be used as a prepurification step preceding solvent extraction. 

More advanced water purification techniques, utilizing UV-irradiation, ozonization, or activated charcoal, may significantly improve the removal of these compounds, but these techniques are not broadly applied due to their high cost. Thus, current European activated sludge treatment plants, with a hydraulic residence time not greater than 14 h, can in most cases not completely eliminate all the estrogens and progestogens from the effluent [23]. 

Figure 5.38. Kinetics of adsorption (biosorption) in biological waste water purification. Time course of chemical and biological oxygen demand expressed as eliminated substrate and degraded substrate following the reaction scheme of substrate degradation and substrate elimination, and evaluation of the rate of adsorption from the difference in chemical and biological rates (Theophilou et al., 1978). The final level Si represents the undegradable substrate. The value of S a represents the maximal capacity of cells (sludge) to adsorb substrate (phenomenon of biosorption ).

The second purpose is to eliminate sands and alluvia which are not wanted in physicochemical purification where they would hinder proper collection, concentration and final disposal of floated oily sludge. 

The raw juice is neutralized and passes through various purification steps. Proteins and lipids from starch flocculate at a suitable pH value and are separated as sludge. Pigments are eliminated with activated carbon and minerals with ion exchangers. The purified juice is evaporated under vacuum (falling-film evaporator) up to a solids content of 70—85%. 

Multiple hearth combustion plants, as shown schematically in Fig. 10.15, are especially suitable for the combustion of moist and paste-like waste. They have been used for decades in the paper industry for energy production from primary and biological sludges, often together with bark. The flue gas purification plant downstream from the multiple hearth furnaces usually consists of wet scrubbers to remove dust and sulfur compounds. The flue gas finally enters the stack via a mist eliminator. 

---