**6. Conclusions and future remarks**

and 0.009 mg P h−1 and 0.011 and 0.006 mg N h−1 for free and immobilized cells. It was observed that the isolated species were more efficient in nutrient recovery than the commercially available strains [120], as the isolated strains were better acclimatized to the prevailing conditions. The anaerobically digested swine manure from a farm digester was used to culture *Chlorophyceae*, *Chlorella sp.*, *Scenedesmus obliquus,* and a cyanobacterium, *Phormidium bohneri*, to evaluate the inorganic nitrogen and orthophosphate removal efficiency. *Chlorella sp.* per‐ formed well in batch cultures wheres *P. bohneri* in semi-continuous conditions [138]. Benthic freshwater algae was also used to recover nutrients from dairy manure [134] in algae growth chambers operated in semi-batch mode by continuously recycling wastewater and adding manure inputs daily. It was found that, when compared to a conventional corn/rye rotation, such benthic algae production rates would require 26% of the land area requirements for equivalent N uptake rates and 23% of the land area requirements on a P uptake basis [134]. Besides microalgae species, some filamentous fungal species also showed some potential to

Harvesting microalgae from treated wastewater is cost intensive, therefore becoming the key to remove and recover the phosphorus. The attached algae cultures for the nutrient removal from the manure waste water was evaluated [140]. It was found that, depending on different culture conditions, the attached algal culture removed 61–79% total N and 62–93% total P from dairy manure. Overall, the attached algal culture removed 62–90% of total phosphorus, 62– 87% of soluble phosphorus, and 43–80% of orthophosphate from dairy manure. The economic assessment of algal turf scrubber technology for treatment of dairy manure effluent showed that economic balance would become more favorable if values from algae as a byproduct and

Aquatic macrophytes are the conspicuous plants that dominate wetlands, shallow lakes, and streams, playing a vital role in healthy ecosystems. Total nitrogen and total phosphorous removal in treatment wetlands can range from 3–98% to 31–99% respectively [142, 143] with an average removal of about 50% [144]. Studies have showed that vascular aquatic plants have acceptable animal feed qualities, ability to remove nutrients from water, and high production rates [145]. Macrophytes constitute a diverse assemblage of taxonomic groups and are often separated into four categories based on their habit of growth: floating unattached, floating attached, submersed, and emergent [146]. Macroscopic flora includes the aquatic angiosperms (flowering plants), pteridophytes (ferns), and bryophytes (mosses, hornworts, and liverworts). Macrophytes based nutrient removal technology has the merits of (1) high productivity of several large-leaf floating plants; (2) high nutritive value of floating plants relative to many emergent species; and (3) ease of stocking and harvesting [147]. Also, the harvested floating macrophytes biomass can potentially be used for composting, soil amendments, anaerobic digestion with methane production, being processed for animal feed, and could be mixed with separated manure solids to increase the amount of nutrients available for exporting off the farm [148]. The biomass can be a good resource of starch, and utilized for the production of value-added products such as fuel ethanol [149]. For example, *Spirodela polyrrhiza* grown on

combine with AD to remove and recover the phosphorus [139].

nutrient trading credits can be realized [141].

534 Biofuels - Status and Perspective

**5.2. Macrophytes for phosphorous recovery**

With the increasing size of livestock farms, especially in the area where livestock raising is highly concentrated, the surplus digested manure applied on soil increases P concentration in agricultural runoff, causing environmental problems like eutrophication. Phosphorus removal and recovery from digested manure reveals its importance in livestock raising area. Coagula‐ tion and electrocoagulation methods have been used for P removal from either digested or undigested animal manure. Compared to the municipal wastewater treatment, the dosing of multivalent cations is more intensive for manure treatment, e.g., the molar ratio of metal to P is mostly more than 3. This can be a result of the presence of high solids content and high level of carbonate/bicarbonate, which may consume additional portion of the added metal coagu‐ lants. Aluminum salts work better than the ferric counterparts in the anaerobic condition of manure media. Electrocoagulation avoids the direct chemical dosing by releasing metal ions through sacrificing metal anode, so is less chemically intensive but consumes additional electric energy. Struvite precipitation is the most commercially available method to recover the phosphorus from manure as fertilizer. The anaerobic digestion of manure seems to be beneficial to the struvite precipitation while more detailed studies are needed. The key issue related to this process is the bioavailability of phosphate in these precipitates, including struvite and hydroxyapatite, to the plant growth. Animal manure contains high level of carbon source in the form of VFAs which can be the ultimate reducing power for PAOs. Therefore, manure can be treated with EBPR process without additional carbon dosing. The high level of ammonium may be simultaneously removed by enriching denitrifying PAOs community with suitable process design. An integrated process combining anaerobic digester and algae cultivation / macrophyte growth can also be an eco-friendly and sustainable process to reduce nutrient loss to environment and to produce valuable biomass.

Overall, these currently available methods for phosphorus removal and recovery are primarily designed for industrial and municipal wastewater treatment, where there is incentive related to the operations. However, the phosphorus removal from the manure is economically challenging because the end products of phosphorus recovery do not justify the cost of the removal process. It is beneficial to combine the AD process with the phosphorus removal and recovery so that the overall techno-economic feasibility of the process can be significantly improved.
