Erhard Hoffmann

Effects of solution conditions on the precipitation of phosphate for recovery: A thermodynamic evaluation

To understand the effects of solution conditions on the precipitation of calcium phosphates from wastewater for recovery, a computer programme PHREEQC was employed to calculate the speciation and saturation-index (SI) with respect to hydroxyapatite of a chemically defined precipitation system, which contains phosphate of 1-200 mg P/l, with Ca/P molar ratios of one to 10 times of the stoichiometric calcium to phosphorus molar ratio of hydroxyapatite, at a pH range of 7.0-11.0. The results show that the SI is respectively the logarithmic function of the phosphate concentration and the calcium concentration, increasing with the increase of either of them; the SI is a polynomial function of the solution pH value and increases with its increase, and the effect of solution pH value is due to its influence on base uptake of the precipitation reaction and the speciation of phosphate and calcium ions; the SI is also a logarithmic function of the solution ionic strength but decreases with its increase; at the temperature range of 5-30 degrees C the SI increases linearly with solution temperature and the effect of temperature is also due to its influence on the speciation of phosphate and calcium ions.

Evaluation of aluminum-silicate polymer composite as a coagulant for water treatment

Aluminum-silicate polymer composite (PASiC), a new kind of inorganic coagulant, was produced by two approaches: (1) hydroxylation of the mixture of AlCl3 and fresh polysilicate (PASiCc); (2) hydroxylated polyaluminum-chloride (PAC) combined with fresh polysilicate (PASiCm). The PASiC products had the following properties: Al2O3 content = 6.40-7.30%, SiO2 content = 0.40-0.82%, Al/Si ratios = 10-20, basicity (OH/Al molar ratio, denoted B) = 1.2-2.0. The coagulation behaviour of PASiC and PAC under conditions typical for coagulation and flocculation in water treatment were investigated by studying the rate of floc size development, the variety of streaming current value, the efficiency of turbidity removal and the effect of pH on the turbidity removal efficiency, and the stability of PASiC. The results detailed in this study suggest that, compared with PAC, PASiC may enhance aggregating efficiency and give better coagulating effects, but weaken charge effectiveness in coagulation process or become unstable when stored for longer time, especially at higher B value and lower Al/Si ratio. The coagulating effect of PASiC is closely linked to the preparation procedure. With the increase of B value and the decrease of Al/Si ratio, the coagulation efficiency of PASiC increases, and at the same B value and Al/Si ratio, PASiCc seems to give a little better coagulation effect than PASiCm but less stability. The Al/Si ratio should not be too low or the B value should not be too high, otherwise, the PASiC products tend to become cloudy or partly gelatinous, which will make them loss some coagulation efficiency.

Effect of humic substances on the precipitation of calcium phosphate

For phosphorus (P) recovery from wastewater, the effect of humic substances (HS) on the precipitation of calcium phosphate was studied. Batch experiments of calcium phosphate precipitation were undertaken with synthetic water that contained 20 mg/L phosphate (as P) and 20 mg/L HS (as dissolved organic carbon, DOC) at a constant pH value in the range of 8.0--10.0. The concentration variations of phosphate, calcium (Ca) and HS were measured in the precipitation process; the crystalline state and compositions of the precipitates were analysed by powder X-ray diffraction (XRD) and chemical methods, respectively. It showed that at solution pH 8.0, the precipitation rate and removal efficiency of phosphate were greatly reduced by HS, but at solution pH > or =9.0, the effect of HS was very small. The Ca consumption for the precipitation of phosphate increased when HS was added; HS was also removed from solution with the precipitation of calcium phosphate. At solution pH 8.0 and HS concentrations <==3.5 mg/L, and at pH > or =9.0 and HS concentrations < or =10 mg/L, the final precipitates were proved to be hydroxyapatite (HAP) by XRD. The increases of solution pH value and initial Ca/P ratio helped reduce the influence of HS on the precipitation of phosphate.

The effect of carbonate on the precipitation of calcium phosphate.

In order to facilitate the recovery of phosphorus from wastewater, the effect of carbornate on the precipitation of calcium phosphate has been studied. The effect of carbonate concentrations up to 5.00 millimoles per liter on the precipitation of 20 milligrammes phosphorus per liter phosphate was studied. At pH 8.00 the precipitation rate of phosphate was greatly retarded by carbonate and the corresponding precipitation efficiency also decreased, but at pH values ≥ 9.00 the effect of carbonate on the precipitation of phosphate was very small. This indicates that carbonate may decrease the precipitation rate and efficiency of calcium phosphate, but the solution pH value is still a key factor influencing the precipitation process. The effect of carbonate on the precipitation of phosphate was attributed to the formation of ion pairs between carbonate and calcium and the decrease of free calcium ions. This resulted in the decrease of the thermodynamic driving force for the precipitation of calcium phosphate, so the effect of carbonate on the precipitation of phosphate was in fact a competing one, although this competing effect was not so obvious at pH ≥ 9.00. Carbonate may be coprecipitated with phosphate from solution, especially at pH 9.00–11.00, and this will decrease the relative phosphorus content of the precipitate. The increases in solution pH value and initial Ca/P ratio are two approaches to overcome the influence of carbonate on the precipitation of phosphate, but the solution pH values should be lower than 10.00 and the initial Ca/P ratios should not exceed 5.00.

Improving the Denitrification Potential in Biological Wastewater Treatment by Dosing Carbon from Sludge Hydrolysis

The stringent demands upon the effluents of wastewater treatment plants in the Fed. Rep. of Germany with regard to the nutrients nitrogen and phosphorus require the upgrading of existing plants applying chemical-biological nutrient removal. Since extensive denitrification presumes an adequate high BOD/N ratio, the dosage of an external carbon source can become essential even for one stage treatment plants.

Upgrading of a co-digestion plant by implementation of a hydrolysis stage.

An existing co-digestion plant needed to be rehabilitated after a 20 year operational period. This was planned to be done in sequence by halving the digester volume for a period of 1.5 years. The aim of the present study was to improve the performance of the halved co-digestion capacity by implementing an upstream thermal hydrolysis reactor or an ultrasonic pre-treatment of the substrates. The results of the ultrasonic bench-scale batch experiments showed that an ultrasonic pre-treatment of the co-substrates ‘municipal bio-waste suspension and excess activated sludge led to disintegration efficiencies of up to 51%. However, treating kitchen-waste and primary sludge in the same manner was not promising as the disintegration yields were rather low. The results of the hydrolysis bench-scale batch experiments showed that the optimal boundary conditions for the hydrolysis reactor were a hydrolysis temperature of about 42 °C at a retention time of 24 h. The results of the continuous two-stage experiments showed that it was possible to reduce the retention time in the second stage to about 24% and to increase the biogas yield to about 12.8 %, and the methane yield to about 28% as a result of the implementation of the hydrolysis reactor in the existing system. After the rehabilitation of the existing digesters it was possible to raise the daily substrate input to the two existing digesters from 312 to 495 m3 day−1 with an upstream hydrolysis reactor volume of only 474 m3.

Acid Extraction of Heavy Metals from Bio-Waste and Bio-Solids

The opportunity to market secondary fertilizers such as bio-wastes and sewage sludges, for example, depends significantly on the pollution load of the final products (e.g. composts, soils) and on the acceptance of people to reuse these products. To ensure a high quality, different kinds of regulations and seals of approval (e.g. Bio-Waste Ordinance) have been passed or introduced in Germany and elsewhere. At present about 90% of the waste treatment operators in Germany can meet these standards, but an increasing amount of separately collected bio-waste will lead to a reduction in raw substrate quality. To support a high recycling rate, payable technologies have to be developed to reduce the pollutant load, especially for highly loaded fractions or charges. Regardless, all efforts have to be undertaken to reduce the amount of pollutants in order to minimise the necessary treatment.