Future trends

Anaerobic digestion of agricultural and food wastes to methane is a mature process that is being used within full-scale facilities worldwide (Table 23.8). Although methane is a relatively low-value product, methanogenic anaerobic digestion still represents the most economically viable technology. Hydrogen production via anaerobic fermentation has the greatest potential as a pre-process step that can be followed by a suitable secondary process step, such as bioconversion of VFAs to other products, such as PHAs,

Table 23.8 Comparison of three biological processing strategies for food wastes treatment

Processing strategy

Culture type

Degree of products separation

Level of maturity

Value added

Anaerobic

Mixed

Easy - gas

Mature, operational

Low to

digestion

on a commercial

medium

scale

Hydrogen

Mixed

Easy - gas

Laboratory phase

Medium

fermentation

Chemical

Pure or

Difficult -

Scale-up phase

Medium to

production

limited

soluble

high

mixture

products

lactic acid and ethanol. Methanogenic anaerobic digestion and hydrogen fermentation use mixed bacterial communities that are selected according to their function. This is well-suited to the non-sterile, complex environment of wastewater treatment. Also, the products from these biological processes can be easily separated as gases and used for electricity generation or other beneficial uses (Table 23.8). Anaerobic digestion has been used for many years to treat wastewaters but, so far, not to produce heat or electricity from hydrogen. With the advancement of fuel-cell technologies, this hydrogen gas can generate electricity in a clean and efficient manner. The anaerobic hydrogen fermentation would use similar hardware to that used currently in methane fermentation anaerobic digesters and the technology has been proven for many years in laboratory and pilot-scale treating using various agricultural and food wastewaters. Of even greater potential is combining hydrogen fermentation with methane fermentation in on-site wastewater treatment systems. However, in our current, cost-effective energy economy, such large-scale implementation may not be economically feasible compared with conversion to more valuable chemical products. Fortunately, specialized high-value biochemical products might soon be produced from food wastewaters as a more economical alternative. The effects of bioreactor design (e.g. IBR, UASB), configuration and operating conditions (e.g. biomass retention, high-rate hydraulic retention) need to be more widely studied and optimized before the processes can be scaled up. More recently, a number of molecular biology techniques have been developed for the qualitative and quantitative analysis of microbial communities from biological processes treating food and agricultural wastes. Monitoring the phylogenic diversity of mixed microbial species may support enhancement of the bioconversion to hydrogen and useful chemicals, and improve understanding of the metabolic characteristics of key microorganisms.

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