Advanced Digestion of Sludge enhances shift of biosolids management strategy in Beijing
Liao, Z.1, Zhang, R.2 and Huang, O.3,1 Cambi Group AS, Norway, 2 Beijing Drainage Group, PR China, 3 Beijing General Design Institute of Municipal Engineering, PR China
(free)
Abstract:
In this paper, the sludge treatment in China is described and the thermal hydrolysis based advanced
digestion of sludge in Beijing for the Three Year Action Plan has been established and described with
five big projects to be completed in 2017. Some data for Gaobeidian plant is analyzed. The advanced
digestion of sludge has gained support in central governmental departments in China, and advanced
digestion is also seen as very beneficial to integrate co-digestion of sludge and biowaste in the next
years.
Introduction:
Statistics from MoHURD (Ministry of Housing and Urban and Rural Development) in China shows that
the number of wastewater treatment plants in cities and towns have increased to approx. 3700 plants
in 2016, with total treatment capacity of approx. 150 million m3/d. The sludge production was 6.25 million
tDS/y in 2013 (Yang et al 2015) and increased to approx. 7 million tDS/y in 2016. It is estimated that
the sludge production will amount to 10 million tDS/y in 2020. Proper sludge treatment and disposal
have been far behind the increase in wastewater treatment capacity due to relatively much less
investment for sludge facilities and increasingly limited space for sludge landfill as one of the traditional
disposal route.
Anaerobic digestion and aerobic composting and land application have been widely recommended in
China as the preferred methods for the treatment and disposal of sewage sludge. However, the output
from sludge treatment by anaerobic digestion or aerobic composting is of poor quality because of the
low organic matter content etc. In 2013, there were about 2600 sludge treatment facilities, but only
about 60 plants had adopted anaerobic digestion processes and just 10 to 30 of them were actually
operating. The treated sludge contains large amounts of nutrients, such as N, P, K, and residual organic
material, and has the potential to be used as soil conditioner and fertilizer during land application. But
the potentially high concentrations of heavy metals, POPs, and grit in the sludge, mainly from industrial
wastewater and rainwater, have greatly limited its use (Feng et al 2015). Such contradiction has resulted
into reluctant application of anaerobic digestion even in some large scale projects.
With the policy change that industrial wastewater treatment should be centralized in industrial parks or
industrial zones, and more industrial parks have been moved out of downtown areas, it is possible to
mitigate the heavy metal and POPs issue to be less critical for sludge from sewage wastewater
treatment in many big cities, such as in Beijing. For example the heavy metal concentration in sludge
from Beijing city has dropped well below the limit numbers in relevant standards for sludge or biosolids
application to land (Zhang et al 2016).
The dryness of final sludge or biosolids cakes after treatment has been one of the critical issues for
disposal in landfill, use in soil remediation, and other uses like brick production, compost use,
incineration, etc. The limitation of achieving higher dryness in traditional treatment processes also
hinders the anaerobic digestion of low VS sludge. In addition, the negative odour and inferior hygienic
condition often hinder the last effort for better land application.
