Background

Present treatments for waste paper

The main way of waste paper processing is to recycle it. As raw material of papermaking, waste paper goes through several technological processes like pulping, deslagging and de-inking, and finally produces new paper. Products include cultural paper, living paper and wrapping paper, etc.

There are also other innovative but restrictive processing methods for waste paper. For example, waste paper can be spread on the farming or agricultural lands to improve the quality of soil; It can be concerted to fuels like ethanol, methane and solid fuels; It can combine with resin to produce artificial timber; It can be break into useful sugars and make animal feeds with some other nutrients…

However, the total consumption of paper and paperboard in the world is 250.217 million tons, and the total amount of waste paper recovered in the world is 82.183 million tons, with a total recovery rate of 47.7%. The data shows a poor recycling rate and the deficiency of present treatments for waste paper:

1. Industrial application of the presently available de-inking technologies is associated with relatively heavy use of de-inking chemicals, which are expensive and environmentally undesirable. The known methods of deinking of waste paper or the like also require heavy use of cleaning and washing equipment which results in the requirement of a high investment capital. The demands for treatment water, energy consumption associated with deinking and cleaning is relatively high. What’s more, the presently available deinking technology has been shown to be inadequate for some furnished, so that the industry is not able to successfully reprocess all of the materials available.

2. Some paper cannot be recycled, because recycling low quality paper may potentially leads to accumulation or un-intended spreading of chemical substances contained in paper. Paper that doesn’t reach the standard of being recycled is burned or buried. Burning pollutes the air while landfill pollutes the soil, threatening the environment.

Present treatments for straws

Straws are by-products from the process of agricultural production, containing large amounts of cellulose that should be reused. Present disposal of straws include directly burning straws, returning straws to the field as organic fertilizer, feeding straws to livestock as fodders and transforming straws to types of energy.

However, according to data provided by the State Council of China, the total production of straws each year is about 900 million, and the recovery rate is lower than 40%. The current treatments for straws are dramatically ineffective, inefficient and are not environmentally friendly. Precisely, the deficiency include:

1. Burning the straws in countryside releases substantial pollutants and dust, which destroy the environment and threaten people’s health after being inhaled. What’s more, the burning of straw is serious waste of the valuable cellulose contained in the straws.

2. Though some local governments have already introduced laws and regulations to ban the burning of straws, the straws are not recycled, but they pile up in the fields, gradually decay and cause soil compaction, which reduces the land fertility and land production capacity.

Cellulose

Cellulose is the main compound of paper and many other wooden products. It is an important structural component of the primary cell wall of green plants and the most abundant organic polymer on Earth and mainly used to produce paperboard and paper. It is an organic compound, a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. The multiple hydroxyl groups on the glucose from one chain form hydrogen bonds with oxygen atoms on the same or on a neighbor chain, holding the chains firmly together side-by-side and forming microfibrils with high tensile strength.

Cellulase

Endoglucanase randomly cut the amorphous region inside the cellulose polysaccharide chain to produce oligosaccharides of different lengths and the end of the new chain.

Exoglucanase acts on the ends of these reductive and non-reductive cellulose polysaccharide chains, releasing glucose or fibrinose.

β-glucosidase hydrolyzes fibrous disaccharides to produce two molecules of glucose.

Microbial Fuel Cells

A microbial fuel cell (MFC) is a bio-electrochemical device that harnesses the power of respiring microbes to convert organic substrates directly into electrical energy. At its core, the MFC is a fuel cell, which transforms chemical energy into electricity using oxidation reduction reactions, relying on living biocatalysts to facilitate the movement of electrons throughout their systems.

Some MFCs require a mediator chemical to transfer electrons from the bacterial cells to the electrode, while we use mediatorless MFCs. The bacteria inside mediatorless MFCs are called exoelectrogens, which are a special class of electrochemically active bacteria that can transfer electrons extracellularly and use a strong oxidizing agent or solid conductor as a final electron acceptor. As a result, these bacteria respire directly into the electrode under certain conditions by using the anode as an electron acceptor as part of its normal metabolic process. Exoelectrogenic activity has been observed in the some strains of bacteria like Clostridium butyricum, Desulfuromonas acetoxidans, Geobacter metallireducens and Escherichia coli, and we finally choose E.coli due to safety consideration.

MFCs play a paramount role in the energy industry. The most immediately foreseeable application of an MFC is in waste water treatment. Microbes love sewage, and the conditions of a waste water treatment plant are ideal for the types of bacteria that can be used in an MFC. Exoelectrogens are more than happy to breakdown and metabolize the carbon rich sewage of a waste water stream to produce electrons that can stream into a cheap conductive carbon cloth anode. The electricity generated from the MFC also offsets the energy cost of operating the plant.

References

[1] Disposal of garbage or garbage and sewage sludge and the like US2246224A[P] 1941-06-17

[2] Method of Treatment of Waste Paper of the Like CA2098092A1 [P] 1992-06-11

[3] Logan B. (May 2009). Exoelectrogenic bacteria that power microbial fuel cells. Nature Reviews Microbiology. 7: 375–383.

[4] 宋成琼. 秸秆综合利用的现状与对策. 农民致富之友. 1003-1650(2014) 11-0104-01.