Environmental Technology

Moriyama, K.; Kojima, T.; Minawa, Y.; Matsumoto, S.; Nakamachi, K.

doi: 10.1080/09593330.2001.9619163pmid: 11804345

An artificial seed crystal material consisting of calcium silicate hydrate (5CaO·6SiO2·5H2O: tobermorite crystals) applicable for phosphorus removal by crystallization was developed. Card-house shaped tobermorite crystals were developed on the seed material where orthophosphate crystallized as a calcium phosphate. The seed material can be manufactured by mixing siliceous and calcareous raw materials, pelletizing and subsequent autoclaving. Laboratory experiments were conducted to apply the new developed seed crystal material in the phosphorus recovery from sludge sidestreams of a wastewater treatment plant. In this crystallization process, the performance of the carbon dioxide degassing process usually carried out when applying crystallization was not necessary, the hydroxyapatite was able to crystallize at a pH of 8.0 to 8.5 without precipitation of calcium carbonates. In the treatment of a sidestream with orthophosphate concentrations of 50 mgl−1 and COD concentrations between 200 to 400mgl−1, phosphorus removal efficiencies ranging from 75 to 85% were observed. The seed crystal material was collected after the laboratory experiments and the chemical estimation and the germination test for agricultural reuse were performed. As a result, it was shown that the hydroxyapatite precipitated on the seed material had a 100% fusibility to soil and had characteristics to be a good nutrient source as a fertilizer for plants.

Jardin, N.; Pöpel, H. J.

doi: 10.1080/09593332208618193pmid: 11804346

Phosphate release and phosphate fixation during sludge treatment of waste activated sludge was investigated with a pilot plant for enhanced biological phosphorus removal, complemented by laboratory investigations of sludge samples from different large enhanced biological phosphorus removal plants. The major part of the eliminated phosphorus in the pilot plant was due to the storage of polyphosphate in the waste activated sludge and was accompanied by an uptake of magnesium and potassium. Stabilising waste activated sludge from the enhanced biological phosphorus removal pilot plant results in a hydrolysis of polyphosphate. As a result of polyphosphate hydrolysis in stabilising systems, phosphate, magnesium and potassium are released, but only potassium remains in solution whereas magnesium and a part of the released phosphate was precipitated as struvite. Another large fraction of the released phosphate was fixed by participation of aluminium.

Çelen, I.; Türker, M.

doi: 10.1080/09593332208618192pmid: 11804347

The effects of environmental conditions on ammonia removal as struvite (Magnesium ammonium phosphate, MAP) were studied in a laboratory scale batch reactor. MAP precipitation was carried out by adding phosphoric acid and magnesium source either as MgCl2 or MgO. The effect of temperature, pH, Mg:N:P ratios were studied. Temperature did not significantly affect ammonia removal between 25-40 °C and over 90% removal was obtained. The effect of pH, however, was significant and highest removal was reached at pH 8.5-9.0. The various stoichiometric ratios of ammonium to Mg and P have been tested and slight excess of Mg and P found to be beneficial for higher recovery of ammonia as struvite. However further increase in Mg and P ratios did not result in further ammonia removal which is also costly for the practical application of the process. When MgO was used as M source, the ammonia recovery was 60-70% whereas the use of MgCl2 has increased this figure up to 95%. In addition a two step purification process was developed to recover MAP crystals from impurities of the anaerobic digester. Firstly, precipitates were dissolved in acid and impurities were removed by centrifugation. The clarified supernatant was re-precipitated by adjusting its pH with caustic. It was shown that in the two steps process white MAP crystals could be obtained with over 85% recovery to be used for another applications. The economical analysis of the process has shown that ammonia in the digester effluents can be recovered at the cost of $7.5-8.0 kg−1 NH4 +-N. The rate of reaction is very fast and is completed almost in minutes. This simplifies the process design resulting in a smaller reaction vessel.

Burns, R. T.; Moody, L. B.; Walker, F. R.; Raman, D. R.

doi: 10.1080/09593332208618190pmid: 11804348

Laboratory and field experiments were conducted using magnesium chloride (MgCl2) to force the precipitation of struvite (MgNH4PO4·6H2O) and reduce the concentration of soluble phosphorus (SP) in swine waste. In laboratory experiments, reductions of SP of 76% (572 to 135 mg P l−1) were observed in raw swine manure after addition of magnesium chloride (MgCl2) at a rate calculated to provide a 1.6:1 molar ratio of magnesium (Mg) to total phosphorus. Adjusting the pH of the treated manure to pH 9.0 with sodium hydroxide (NaOH) increased SP reduction to 91% (572 to 50 mg P l−1). X-ray diffraction of the precipitate recovered from swine waste slurry treated only with MgCl2 confirmed the presence of struvite. The molar N:P:Mg ratio of the recovered precipitate was 1:1.95:0.24, suggesting that compounds in addition to struvite were formed. In a field experiment conducted in a swine manure holding pond, a 90% reduction in SP concentration was observed in approximately 140 000 l of swine manure slurry treated before land application with 2000 l MgCl2, (64% solution) at ambient slurry temperatures ranging from 5 to 10°C.

Parsons, S. A.; Wall, F.; Doyle, J.; Oldring, K.; Churchley, J.

doi: 10.1080/09593332208618188pmid: 11804349

Struvite in wastewater treatment plants was identified as early as 1939. Problems with struvite formation date back to the 1960s when it was noticed at the Hyperion treatment plant, Los Angeles. Operators at the plant noticed crystalline deposits on the underside of post digestion screens. The digested sludge stream was diluted and it was thought the problem was solved, until five years later when the normal gravity flow of digested sludge had decreased to such a stage that pumping was required. This paper reports the findings of a series of experiments undertaken to identify the potential of recovering struvite from sludge liquors. Seven sludge treatment works (STW) have been investigated including one detail. A number of the works has the potential to form over 100 mg l−1 of struvite.

Wentzel, M. C.; Musvoto, E. V.; Ekama, G. A.

doi: 10.1080/09593332208618189pmid: 11804350

A three phase (aqueous/solid/gas) mixed weak acid/base kinetic model developed by Musvoto et al. is applied to simulate the physical and chemical processes that occur on aeration of anaerobic digester liquors. Included in the model are the kinetic reactions for (i) weak acid/base dissociations (water, carbonate, ammonium, phosphate, and short-chain fatty acids), (ii) precipitation of struvite, newberyite, amorphous calcium phosphate, calcium and magnesium carbonate, (iii) ion pair formation and (iv) stripping of CO2 and NH3 gases. To generate data for model application, batch aeration tests were conducted on two anaerobic digester liquors from (i) a spent wine upflow anaerobic sludge bed (UASB) digester and (ii) a sewage sludge anaerobic digester. In the batch tests pH, Ca, Mg, PO4-P, free and saline ammonia (FSA) and H2CO3* alkalinity (from which inorganic carbon is calculated) were measured. After establishing from the literature values for (i) weak acid/base equilibrium constants (pKa), (ii) weak acid/base kinetic rate constants (Kra), and (iii) ion pair stability constants (pKST), and trial and error determination of (iv) mineral solubility products (pKSP) (within the range reported in the literature), (v) ion pair kinetic rate constants (KrIP), (vi) mineral precipitation rate constants (Kppt) and (vii) gas stripping rates (KrG), a good correlation between predicted and measured data was obtained for all the parameters for both liquors. The solubility product values for the minerals that precipitated were the same for both liquors and fall in the range of values quoted in the literature, but the specific precipitation rate constants of the minerals differed for the two liquors.

Suschka, J.; Machnicka, A.; Poplawski, S.

doi: 10.1080/09593330409355432pmid: 11804351

Recovery of phosphates from ferrous and /or ferric residues as a result of chemical precipitation was investigated. The product of biological reduction of sulphates – sulphides and free hydrogen sulphide – can react with iron ions forming ferrous sulphide which are less soluble then ferrous phosphates, releasing phosphates into solution. Laboratory scale experiments have confirmed the possibility of phosphates recovery. It was found that strict anaerobic conditions are required and that the process is COD/S ratio dependent.

Webb, K. M.; Ho, G. E.

doi: 10.1080/09593330.2001.9619170pmid: 11804352

A number of waste solutions from processes operating in Western Australia (anaerobic digester supernatants, facultative lagoon treated piggery and abattoir waste effluents) were characterised chemically and by automated titration to determine acid-base characteristics. Titrations were over the pH range 2-12 or less. All waste solutions were excess molar ratios of NH3-N to PO4-P (5:1 up to 20:1). The amounts of acid or base reagent required for each waste solution type were in the sequence anaerobic digestor supernatant>>piggery effluent>abattoir effluent. The most efficient removal of N and P of the field samples considered is from piggery effluent. The results indicate conditions for optimising the removal of N and P by precipitation (predominantly struvite) as well as the way forward in determining the full scope of N and P waste streams for which recycling by precipitation (either magnesium or calcium based salts) may be feasible.

Liberti, L.; Petruzzelli, D.; De Florio, L.

doi: 10.1080/09593330409355443pmid: 11804353

Nutrients control technologies from wastewater are based on destructive technologies which defer the problem from the diluted liquid-phase (effluent) to a more concentrated waste (sludge) in the case of phosphates, or to nitrogen gas and/or volatile compounds in the case of ammonia. The REM NUT process allows for simultaneous removal of phosphate and ammonium ions by selective ion exchange and recovery by chemical precipitation in the form of struvite (magnesium ammonium phosphate) from the spent exchangers regeneration eluates. In the paper revised versions of the REM NUT process, i.e., P-driven layouts, are presented and cost effectiveness is compared to chemical precipitation based on the use of ferric chloride.

Van Der Houwen, J. A. M.; Valsami-Jones, E.

doi: 10.1080/09593332108618187pmid: 11804354

This paper describes current knowledge of phosphate precipitation chemistry in the context of phosphorus recovery from wastewaters, and presents experimental results on the effect of organic species, as key potential inhibitors, to the precipitation of calcium phosphate. The supersaturation required for precipitation at 25°C, pH 7, 0.1 M ionic strength and near-stoichiometric (for hydroxylapatite) calcium to phosphate molar ratio was determined under spontaneous precipitation conditions. The experiments were carried out in air. The phase precipitating at the critical concentration was allowed to grow under constant supersaturation. The influence of organic ligands on the precipitation was investigated using two small molecular weight organic ligands, acetate and citrate, present at a concentration of 10−3 M. The precipitate was studied using X-ray diffraction and scanning electron microscopy. Good reproducibility of the experiments, which were carried out in triplicate, was observed. The study assessed the supersaturation necessary for spontaneous precipitation of hydroxylapatite to be 10.93, calculated using a solubility constant of log K=−57.74. The required supersaturation was not affected by the presence of acetate. However, citrate was found to increase the supersaturation required for precipitation to 11.73. It is likely that this increase is due to binding of citrate on the active growth sites of newly formed nuclei, thereby inhibiting precipitation. All experiments showed formation of a single phase: micro-crystalline hydroxylapatite.