Ayla Uysal

The determination of fertilizer quality of the formed struvite from effluent of a sewage sludge anaerobic digester

The formation of struvite (MgNH(4)PO(4).6H(2)O) in wastewater treatment plants can lead to scaling and thus operational problems reducing the treatment efficiency. However, struvite has significant commercial value as an agricultural fertilizer. Therefore, controlled struvite formation in wastewater treatment plants not only presents an opportunity to recover nutrients but also corresponds to the valorization of wastes. NH(4)-N and PO(4)-P removal and recovery from the effluent of a full-scale sewage sludge anaerobic digester via controlled struvite precipitation were investigated in this study. The effect of the residual heavy metal and micropollutant content of the formed struvite on fertilizer quality was also evaluated. Removal efficiencies of NH(4)-N, PO(4)-P and COD were 89.35%, 95% and 39.78% when Mg:N:P molar ratio was 1.5:1:1 and pH was 9.0. Mercury, nickel, zinc and chrome concentrations derived from struvite precipitation were below the regulatory limit for fertilizer usage in Turkey. The precipitate did not contain polychlorinated biphenyls (PCB). X-ray diffraction (XRD) analysis conducted on the precipitate indicated a struvite formation.

Recovery of nutrients from digested sludge as struvite with a combination process of acid hydrolysis and Donnan dialysis

In this study, the effects of the combination of acid hydrolysis and Donnan dialysis on the high-quality struvite production from digested sewage sludge were examined. The Box-Behnken design was applied in the hydrolysis using oxalic acid for the optimization of the conditions that affect the nutrients and metal release from digested sludge. An optimal condition was obtained at oxalic acid 0.5 M, acid/sludge ratio (mL/g) 10/1, and reaction time 60 min. The separation of the metals from hydrolyzed sludge liquid was carried out with the Donnan dialysis using a Nafion 117 cation exchange membrane. At the end of the 4 hours of operating time, the recovery values for Zn, K, Na, Mg, Fe, and Al were obtained as 67.9%, 62.1%, 57.6%, 39.4%, 5.3%, and 2.5%, respectively. The produced struvite meets the legal limits for fertilizer use in terms of Cd, Cu, Ni, Pb, Zn, Hg, and Cr content specified by Turkish regulations. A high Ca/Mg molar ratio and the presence of K, Na, and Al affected the efficiency of struvite crystallization. The use of oxalic acid resulted in low Ca release, and the metal ions that affect struvite purity were eliminated using Donnan dialysis.

Optimisation of nutrients and metals release from municipal sewage sludge by chemical extraction using Box-Behnken design

In this study, H2SO4 and citric acid using Box-Behnken design were utilised in order to release the nutrients and metals fixed in the anaerobic digested and dewatered sludge. For the dewatered sludge, release of 16.27 mg/L P and 120.74 mg/L NH4+ were obtained at optimised conditions with citric acid 0.3 M, H2SO4 6% and reaction time 105 min, respectively. For the digested sludge, release of 764.1 mg/L P and 388.3 mg/L NH4+ were achieved at optimised conditions with citric acid 0.1 M, H2SO4 6% and reaction time 180 min, respectively. In all experimental conditions, Fe and Zn were released together with nutrients. For digested sludge, a minimum Fe and Zn release was achieved at optimised conditions with citric acid 0.5 M, H2SO4 2% and reaction time 105 min, respectively. Thus, this test conditions are thought to be more appropriate for the nutrients recovery from digested sludge.