**3. Influence of organic matter content**

The particulate and colloidal organic matter is known to slow the kinetics of struvite formation. Even a low concentration of organic matter has an adverse, inhibiting effect on crystal growth [31, 33–35]. To this end, some promising results for the separation of the nutrients ions from the organic matter dissolved and suspended in the wastewater streams were already obtained in our recent studies [36]. Membrane separation was used as a process to improve wastewater characteristics for further struvite crystallization. The most commonly used membrane processes for organic matter separation are Microfiltration (MF) and Ultrafiltration (UF). It was found that if dead-end MF membranes are used a fast membrane permeability decrease and

**Figure 7.** *Membrane filtration unit [32].*

sharp loss of phosphates is observed (case study for filtration of municipal WWTP sludge dewatering liquid and swine). Positive results were obtained by the application of ceramic ultrafiltration membranes (**Figure 7**). The UF unit has been operated in a cross-flow mode at a pressure of up to 6 bars and a recirculating flow rate of 20%. The UF module was loaded with sludge water with the following characteristics: COD – 78.1 gO2/l and TSS - 2.6 g/L; phosphorus and ammonia levels of 250 mg/L and 2.4 g/l, respectively. Two types of ceramic tube membranes, namely with pore sizes of 50 nm and 200 nm, were used [32].

The volumetric rate of the permeate stream did not change during all filtration runs. In the case of usage of a 200 nm UF module, the permeate parameters were: phosphates −195 mg/l, COD - 642 mgO2/L, TSS - 0.15 g/L, and ammonium ions - 224 mg/l. The organic matter and TSS removal rates observed were over 99% while the phosphate reduction was as low as 22% (**Table 3**). In the case of the 50 nm UF module, the removal efficiencies for COD and TSS were again over 99%, however, the phosphates in the filtration product were much lower (only 48% of the initial total amount). In summary, the results obtained show that the 200 nm UF module is more appropriate as it keeps a higher level of phosphorus in permeate, i.e. higher recovery and struvite yield is expected.


#### **Table 3.**

*Results obtained through the application of the nano-filtration process.*

*Phosphorus Recovery through Waste Transformation: Implication for an Alternative Fertilizer DOI: http://dx.doi.org/10.5772/intechopen.111856*
