Anaerobic digestion (AD) is commonly used for the stabilization of agro-food wastes and recovery of energy as methane. Since AD removes organic C but not nutrients (N and P), additional processes to remove them are usually applied to meet the stringent effluent criteria. However, in the past years, there was a shift from the removal to the recovery of nutrients as a result of increasing concerns regarding limited natural resources and the importance given to the sustainable treatment technologies. Recovering N and P from anaerobically pretreated agro-food wastes as easily transportable and marketable products has gained increasing importance to meet both regulatory requirements and increase revenue. For this reason, this review paper gives a critical comparison of the available and emerging technologies for N and P recovery from AD residues.

Struvite crystallization (MgNH(4)PO(4)·6H(2)O, MAP) could be an alternative for the sustainable and economical recovery of phosphorus from concentrated wastewater streams. Struvite precipitation is recommended for those wastewaters which have high orthophosphate concentration. However the presence of a cheap magnesium source is required in order to make the process feasible. For those wastewater treatment plants (WWTP) located near the seashore magnesium could be economically obtained using seawater. However seawater contains calcium ions that could interfere in the process, by promoting the precipitation of amorphous magnesium and calcium phosphates. Precipitates composition was affected by the NH(4)(+)/PO(4)(3-) molar ratio used. Struvite or magnesium and calcium phosphates were obtained when NH(4)(+)/PO(4)(3-) was fixed at 4.7 or 1.0, respectively. This study demonstrates that by manipulating the NH(4)(+)/PO(4)(3-) it is possible to obtain pure struvite crystals, instead of precipitates of amorphous magnesium and calcium phosphates. This was easily performed by using either raw or secondary treated wastewater with different ammonium concentrations.

Sludge is a by-product of municipal wastewater treatment plants (WWTPs) and its management contributes significantly to the operating costs. Large WWTPs usually have anaerobic sludge digesters to valorize sludge as methane and to reduce its mass. However, the low methane market price opens the possibility for generating other high value-added products from the organic matter in sludge, such as polyhydroxyalkanoates (PHAs). In this work, the economic feasibility of retrofitting two types of WWTPs to convert them into biofactories of crude PHAs was studied. Two cases were analyzed: (a) a large WWTP with anaerobic sludge digestion; and (b) a small WWTP where sludge is only dewatered. In a two-stage PHA-production system (biomass enrichment plus PHAs accumulation), the minimum PHAs cost would be 1.26 and 2.26 US$/kg PHA-crude for the large and small WWTPs, respectively. In a single-stage process, where a fraction of the secondary sludge (25%) is directly used to accumulate PHAs, the production costs would decrease by around 15.9% (small WWTPs) and 19.0% (large WWTPs), since capital costs associated with bioreactors decrease. Sensitivity analysis showed that the PHA/COD (Chemical Oxygen Demand) yield is the most crucial parameter affecting the production costs. The energy, methane, and sludge management prices also have an essential effect on the production costs, and their effect depends on the WWTP’s size.

Cellulose from used toilet paper is a major untapped resource embedded in municipal wastewater which recovery and valorization to valuable products can be optimized. Cellulosic primary sludge (CPS) can be separated by upstream dynamic sieving and anaerobically digested to recover methane as much as 4.02 m/capita·year. On the other hand, optimal acidogenic fermenting conditions of CPS allows the production of targeted short-chain fatty acids (SCFAs) as much as 2.92 kg COD/capita·year. Here propionate content can be more than 30% and can optimize the enhanced biological phosphorus removal (EBPR) processes or the higher valuable co-polymer of polyhydroxyalkanoates (PHAs). In this work, first a full set of batch assays were used at three different temperatures (37, 55 and 70 °C) and three different initial pH (8, 9 and 10) to identify the best conditions for optimizing both the total SCFAs and propionate content from CPS fermentation. Then, the optimal conditions were applied in long term to a Sequencing Batch Fermentation Reactor where the highest propionate production (100-120 mg COD/g TVS·d) was obtained at 37 °C and adjusting the feeding pH at 8. This was attributed to the higher hydrolysis efficiency of the cellulosic materials (up to 44%), which increased the selective growth of Propionibacterium acidopropionici in the fermentation broth up to 34%. At the same time, around 88% of the phosphorus released during the acidogenic fermentation was recovered as much as 0.15 kg of struvite per capita·year. Finally, the potential market value was preliminary estimated for the recovered materials that can triple over the conventional scenario of biogas recovery in existing municipal wastewater treatment plants.