Methane yield, VS reduction, OLR, and HRT are important operating parameters for the ADP. Generally, the ratios of actual to theoretically calculated methane yield and VS reduction are relatively low (typically from 0.4 to 0.6). Biodegradability is often limited by the ability of anaerobic bacteria to hydrolyze complex organic compounds as well as by the slow rate of acetogenesis and methanogenesis stages of AD. A variety of methods, including process parameters and reactor design, feedstock pretreatment and conditioning, and source and modification of anaerobic microorganisms are used to increase the AD efficiency. The general principles of digestion with non-algal feedstock can be applied to the AD of algae. Environmental parameters have a significant impact on AD performance. Optimal process design and control allow enhancing methane yield, increasing OLR and decreasing HRT.

Reactor Design

The main goal of optimal reactor design is achievement of the maximum methane yield at high OLR and low HRT in order to reduce reactor volume and capital costs. Several high-rate digester configurations were developed over the past several decades for digestion of biosolid wastes and residues. Their characteristics and

advantages and disadvantages are described elsewhere [68, 70, 371-373]. One major strategy is decoupling HRT from SRT by anaerobic sludge immobilization [374-379], granulation, and floc formation [380-387], biomass recycling [388], or membrane retention [389-391] . The methane yield and methane production in a nonmixed vertical flow reactor (NMVFR) digester were larger and more stable at higher OLR compared to CSTR (Fig. 14) [79]. Another promising approach applied for biosolids digestion is separation of the hydrolysis and acetogenesis steps from methanogenesis, a process called a two-stage system [392-394]. A two-stage anaer­obic reactor system achieved stable methane production from M. pyrifera and

D. antarctica with an HRT of one day for each stage [395].

A special case of the two-stage system is preliminary treatment of macroalgae in percolation reactors with natural hydrolysis and acidogenesis processes. In percola­tion reactors, algae are stored in a tank yielding a drained liquid product containing VFAs and ethanol as good substrates for methanogenesis [246, 248] . Legros and colleagues compared maximum OLRs for three systems: one-step CSTR, two-step CSTR, and percolator followed by upflow and fed batch digesters for liquid and solid phases, respectively. The reported maximum volumetric loads were 2.5 g VS/L-day for one-step, 4 g VS/L-day for two-step systems, 5.3 g VS/L-day for upflow reactor, and 6.3 g VS/L-day for fed batch digester (assuming COD/VS equal 1.25). A comparison of methane yield and production rate from Ulva and Ulva juices is presented in Fig. 11.

The AD of hydrolysis juices is more economically efficient compared to diges­tion of whole macroalgae due to lower reactor size, energy for substrate heating, grinding, and pumping [246-248]. For example, the volume of digester with fixed bacteria for digestion of hydrolysis juices is 25 times smaller compared to a CSTR digester required for whole algae [245] .

Mixing

Mixing is essential for optimal distribution of substrate, intermediate products, nutrients, and microorganisms throughout the anaerobic digester, but excessive mixing can be unfavorable for methanogenic bacteria [396-399]. Furthermore, proper mixing diminishes temperature and concentration fluctuations.

Temperature Conditions: Psychrophilic vs. Mesophilic vs. Thermophilic

Psychrophilic conditions were found to be nonfavorable for methane production from algae even when natural inoculum adapted to low temperature is used to start the AD process [77, 111, 400]. The influence of temperature on digestion of Enteromorpha Iinza, Enteromorpha intestinalis, Enteromorpha prohfera, and P percursa in the range from 10 to 35°C is presented in Fig. 15 [77]. Insignificant methane was detected at temperatures under 25°C. The final methane yields were identical at 30 and 35°C, but the initial methane production rate was significantly higher at 35°C.

Generally, the degradation rate of biosolids is usually higher under thermophilic conditions, reducing the volume of the anaerobic digester. Furthermore, raising the temperature increases the lipid solubility in water and its availability for enzymatic attack. For example, during the digestion of a mixture of Scenedesmus spp. and Chlorella spp., a 27% higher methane yield was observed at 50°C compared to 35°C [110]. Mesophilic conditions can favor algal survival in the anaerobic digester and make them more resistant to biodegradation. This is evidenced by the low meth­ane yield from the digestion of cyanobacteria at 22.3°C [401 ]. The intact Scenedesmus cells were detected in a digester after 6 months of incubation [157].

But, operating under mesophilic conditions is generally more stable due to lower sensitivity of mesophilic organisms to temperature and substrate variations. Moreover, toxicity from ammonia and salts increases as the temperature increases. The observed methane yield from M. pyrifera at thermophilic conditions was approximately two times smaller than under mesophilic conditions [402]. A. max­ima gave 40-80% larger methane yield at mesophilic conditions compared to thermophilic regimes (Fig. 16b) [158]. The methane production from A. maxima was completely inhibited at 15 and 52°C (Fig. 16a) [160]. Keenan reported a similar methane yield for mesophilic and thermophilic regimes (0.312 and 0.318 L/gVS) during AD of Anabaena flos-aquae at OLR between 3.2 and 2.8 gVS/L-day [403].