Climate Change Affected by Sludge Production

 Wastewater treatment plants generate massive amounts of waste activated sludge due to extensive produced the consumption of feedstock with the addition of increasing population worldwide. Average of annual sludge production output regards to the countries such as Germany, England, France and America, respectively, 22, 12, 8.5 and 71 MT. China is no different with 20 MT annual sludge production. [1]

 In addition to these, total sludge production in China can be an example, 6.25 million tons of dry solids were produced in 2013 with an average growth rate of 13% from 2007 to 2013, and the total amount might exceed 14 million tons (dry solid) in 2020. [3] The WAS specifically contains high levels of organic compounds in forms of extracellular polymeric substances (EPS) and cellular organic matter, and requires sludge treatment to reduce organic contents and pathogen contamination prior to its ultimate disposal. [2]

 As effective methods in WAS treatment, which are able to remove toxic substances in sludge such as pathogenic organisms, heavy metals, micro-pollutants and emission sources. In China currently, thickening, dewatering, anaerobic digestion and composting is used with different percentages as being the highest thickening and the lowest is composting. As an end-of life treatment, generally landfill, agricultural use or incineration is applied. [1]

According to a cost combined life-cycle assessment report about a sludge treatment facility in China, scenario results have indicated that anaerobic digestion reduces the economic burden since it also reduces the dry mass volume ofsludge and energy recovery requirement. On the other hand, landfill as highest and incineration technologies as lowest have contributed to environmental burden. In terms of emissions of heavy metals that landfill and incineration technologies generated, they have contributed the marine and human toxicity. However, if the energy recovery can be applied, then the environmental and economic burden of these systems are diminished. [1]

In the scope of energy consumption amounts of possible treatment processes, anaerobic digestion seems to have abilities to regenerate electricity and heat by using methane produced by sewage sludge. Without considering the energy recovery from landfill and incineration processes, anaerobic digestion can be the most environmental friendly sludge treatment process depending the life-cycle assessment research conducted by Changqing Xu et.al,. 5 In contrast to the emission contribution to global warming due to methane and other greenhouse gases that can generate in sludge, this study have conducted through taking into account the energy recovery point of processes only. Therefore, agricultural spreading and the fluidized beds are also included in the attractive solutions for huge energy consumption of sludge management facilities. [1]

 

Anaerobic digestion is actually the worst scenario (highest impact) option because of its direct greenhouse gas contributions. However, the gas can be utilized as it can be seen in Figure 1. 

Figure 1. Anaerobic Digestion -Biogas Utilization

On the other hand for the other methods that deal with waste, landfill creates highlevel terrestrial contamination and direct human toxicity unlike anaerobic digestion. Gravity thickening and dewatering have indicated no significant environmental burden at all as an output of this LCA. In the scope of terrestrial contamination, again, agricultural use have high contribution.[ 3] Incineration contributed greatly to ozone depletion and photochemical pollutant formation. If the energy recovery and carbon fixation can be efficiently carried out, then apparently, the greenhouse gas emission due to anaerobic digestion will be reduced as well. [1]

Figure 2. Incineration

As a conclusion, researchers are aiming to find a technology or invent one that is energy efficient as well as creates less greenhouse gases or preferably do not produce greenhouse gas at all. Anaerobic digestion seems to have the attractive possibilities, which are close to the desired technology because of energy recovery, biohazard control and its capability of stabilizing organic substances. However, the biodegradability of the substances that sludge contain is very significant matter since the efficiency of the process as well as the output greenhouse gas depends on that. The more the greenhouse gas produced controllably, the more energy can be recovered result in higher efficiency and sustainability.

 

References

 [1] P. W. A. J. M. K. S. Adriana Maria Lotito, "Nitrous oxide emissions from the oxidation tank of a pilot activated sludge plant," Water Research, no. 46, pp. 3563-3573, 2012.

 [2] W. C. J. H. Changqing Xu, «Life-cycle environmental and economic assessment of sewage sludge treatment in China,» Journal of Cleaner Production, no. 67, pp. 79-87, 2013.

[3] I. F. C. A. H. B. J. G. Enrica Uggettia, «Carbon footprint of sludge treatment reed beds,» Ecological Engineering, no. 44, pp. 298-302, 2012.