**4. Case studies of struvite production**

The most common methods for P-removal from domestic wastewater treatment are Chemical Phosphorus Removal (CPR) and enhanced biological P removal (EBPR) [37]. Currently, the biological process for phosphate removal is more attractive as the rejected sludge liquor after digestion units have a higher phosphorus content with higher phosphorus recovery. In addition, the CPR technique has the disadvantage of an increased sludge volume (by 26% compared to EBPR) with reduced dissolved P content. However, in several cases, the less complicated approach, the CPR, is still the preferred approach, mainly due to its high efficiency in achieving the requirements for residual P in the WWTP outlet streams. Contrary, even taking into account its higher sustainability, the EBPR can rarely provide P removal efficiency higher than 60%. It is well recognized that the combination of EBPR and anaerobic sludge digestion offers a better opportunity for P extraction because, during the anaerobic stage, P is released to a high extent and makes the following struvite precipitation more efficient. The eventual struvite precipitation after the combination of chemical and biological phosphorus removal is shown herein by two case studies aiming at the WWTP of Burgas City in Bulgaria (applying both chemical and biological P removal) and the municipal WWTP in the town of Pomorie. Bulgaria which applies solely biological process for P removal. Both plants are serving settlements located on the South-East Coast of the Black Sea. The process used for both is a conventional activated sludge system with denitrification/nitrification zones (**Figure 8**). The conditions in this system configuration allow partial bio-dephosphatation. However, in the case of Burgas WWTP, FeCl3 is added before the activated sludge basin (doses ranging from 1.7 to 2.4 mgFe3+/mgP) for the chemical precipitation of phosphates. This method has one additional drawback i.e. locking the phosphates into the insoluble form of FePO4, which needs to be treated for re-mobilization of the phosphate ions.

The data shown in **Table 4** reveals that a considerable part of phosphates remain in the returned streams and are available to be recovered.

**Figure 8.** *Principle scheme of the WWTPs under study.*


#### **Table 4.**

*Phosphorus balance.*


#### **Table 5.**

*Phosphate and ammonia levels in the centrate.*

Based on the average phosphate and ammonia levels in the centrate, the amount of struvite produced in the corresponding plants has been calculated (**Table 5**). The struvite production estimation is based on experimental results obtained for optimal Mg/P molar ratios and pH 9 when seawater brine is used as a source of magnesium ions (according to the characteristics mentioned above in the text).
