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In the present study, phosphorus removal was studied using as coagulant spent alum sludge from a water treatment plant of EYDAP (Athens Water Supply and Sewerage Company) and compared to alum (Al2(SO4)3.18H2O), iron chloride (FeCl3.7H2O), iron sulfate (Fe2(S04).10H2O) and calcium hydroxide (Ca(OH)2) at a constant pH (equal to 6). The comparison was based on their efficiency to remove phosphorus in synthetic wastewater consisting of 10 mg/L P as potassium dihydrogen phosphate and 50 mg/L N as ammonium chloride, The experiments were carried out using a jar-test apparatus and the measurements were performed according to the Standard Methods for the Examination of Water and Wastewater. Pure alum, iron chloride and iron sulfate were much more efficient in phosphorus removal than the spent alum sludge but in the case of calcium hydroxide, phosphorus removal was very low in pH = 6. Specifically, orthophosphate were totally removed by alum using 15 mg/L as Al, by alum sludge using 75 mg/L as Al and by FeCl3.7H2O or Fe2(SO4).10H2O using 30 mg/L of Fe while in the case of calcium hydroxide P removal was actually zero. pH measurements showed that the uptake of phosphates is associated to the release of OH ions in the solution and that the end of P uptake is accompanied by the stabilization of pH. Finally this spent alum sludge was tested on municipal wastewater and proved to be effective as apart from phosphorus it was shown to remove turbidity and COD.

The stripping of n-butyl mercaptan (RSH) from neutral and alkaline aqueous solution with gaseous nitrogen is investigated at ambient temperature and pressure in a semibatch system. A model considering the dynamic state, the thiol mass balance, the effect of RSH dissociation, and the deviation from chemical equilibrium is developed and solved numerically. The assumption of chemical equilibrium between undissociated thiol and its ion (RS -) is valid at neutral pH, but at alkaline conditions, RS -f RSH is the rate controlling step, with k ) 5 × 10 -6 s -1 at 301 K and pH 11.7. The effectiveness of inert gas stripping is mainly governed by thiol's hydrophobic nature and its neutral/ionic form speciation. At pH values below 8.5, thiol concentration falls in an exponential form, whereas at pH values above 10.5, it tends toward a linear form. The mass balance model is extremely sensitive to uncertainty in γ ∞ , whereas k governs thiol emission rate out of the liquid system.

In the present study freshly precipitated spent alum sludge [15-75 mg Al l(-1)] and alum [3-15 mg Al l(-1)] were tested and compared for their efficiency to remove phosphorus in synthetic wastewater [10mg P l(-1), 25 degrees C, pH=6] and it was proven that pure alum is much more efficient at phosphorus removal than the spent alum sludge. The effect of the pH on phosphorus removal efficiency was studied and it was proven that maximum orthophosphate removal is achieved in pH values 5-6 in both cases, alum and spent alum sludge, although alum is efficient in a wider pH range (4-7). The effect of aging [up to two months at 25 degrees C] on the efficiency of the spent sludge to remove phosphorus in synthetic wastewater was studied and it was proven that this efficiency is decreased through aging (up to 20% for 2 months aging). X-ray diffraction was used in order to examine whether this decrease is caused by changes in the structure of adsorbent and a mechanism was proposed to explain the above changes. Kinetics was investigated in the case of fresh and aged spent alum sludge and the pseudo-second-order chemisorption rate equation fits satisfactorily the experimental data [k: 0.286-1.048 g mmol(-1) min(-1), at 23-26 degrees C]. Finally, equilibrium was studied for fresh and aged spent alum sludge and the Freundlich isotherm fitted best the experimental data suggesting heterogeneous sorption [K(F)=14.4-19.7, N: 0.13-0.200].