Chapter 2: Literature Review
2.4 Production of SBAC
2.4.1 Physical activation for production of SBAC
Physical activation of sewage sludge is done by heating dried or semi-solid sludge at high temperatures in an inert environment or in the presence of N2, CO2, or steam. This process is known as pyrolysis (Gao et al., 2020). The BET surface area of activated carbon produced through pyrolysis ranges from 30-200 m2/g (Smith, 2009). The factors affecting sewage sludge's physical activation are pretreatment conditions, temperature, dwell time, atmosphere, and post-treatment conditions (Méndez et al., 2009; Monsalvo et al., 2011).
Pyrolysis can be fast or slow depending on the temperature and dwell time and can be catalytic or non-catalytic. Depending on the temperature and other operating conditions, pyrolysis can produce char, bio-oil, or syngas (Gao et al., 2020; Sanz-Santos et al., 2021).
A summary of recent studies that used physical activation to convert sludge into activated Raw material
Sewage sludge/
Industrial sludge
Physical activation/
chemical activation Acids: H2SO4, HNO3, H3PO4 Bases: NaOH, KOH
Salts: ZnCl2, KCO3
Pyrolysis Pyrolysis temp: 350-
1000 °C Gases: N2,CO2, air,
NH3
Pyrolysis time: 0.5-6 h
Post-pyrolysis treatment HCl, NaOH, distilled water Sludge-based
activated carbon (SBAC) Physical characterization SEM/TEM, moisture and ash content, pore volume, density, X-
ray absorption
Chemical characterization
pH, elemental analysis, BET, FTIR, CEC, GC, NMR, point zero charge analysis
Applications Pharmaceuticals,
heavy metals, organic dyes,
phenolic compounds
carbon is given in Table 1. The results indicate that a higher activation temperature, CO2
atmosphere, and steam activation yield better-quality activated carbons (Table 1).
Méndez et al. (2005) used low-temperature pyrolysis to obtain SBAC of more than 100 m2/g surface area. They used different sludge materials (aerobic and anaerobic) to produce adsorbent materials. The carbonization was carried out at 450℃ with a dwelling time of 1 h. The samples with the highest carbon material were then activated at 250℃ for 4 h in the presence of air. The aerobic sludge produced a SBAC of 102 m2/g surface area, whereas anaerobic sludge produced a SBAC of 105 m2/g. The produced SBAC was tested for metal adsorption and was proven successful for iron removal from water.
Rio et al. (2006) produced SBAC using physical activation in the presence of steam using municipal wastewater sludge. They obtained viscous liquid sludge from the treatment plant with 39.4% of carbon. In the presence of nitrogen gas, carbonization of sludge was done at 600℃ for 1 h. The activation of carbon was done at higher temperatures of 750-850℃ and an activation time of 30-90 min with a steam flow rate of 2.5 L/min. Rio et al. also tested the adsorption capacity of the obtained carbon for metal ions, phenol, and dyes. The BET surface area of the SBAC obtained ranged between 144- 228 m2/g. The highest surface area of the SBAC was obtained at 763℃ and activation time of 39 min with mesoporous properties, and showed significant ability for dye removal.
Barry et al. (2019) investigated the impact of pyrolysis rate and temperature on the conversion of sewage sludge into char (activated carbon) and bio-oil. Their study concluded that a higher pyrolysis temperature of 500℃ is better suited for bio-oil production. In contrast, slow pyrolysis at 400℃ is better suitable for char production as it yields higher quality and more stable char than fast pyrolysis.
Godlewska et al. (2019) analyzed the impact of pyrolysis temperatures and the addition of biomass on physically activated SBAC produced in the presence of either N2
or CO2. They produced biochar using sewage sludge at pyrolysis temperatures of 500, 600, and 700℃ using a slow pyrolysis method and a dwell time of 3 h. The oxygen-free environment was maintained using N2 or CO2. They analyzed the surface area, porosity, polarity, and hydrophobicity of the produced bio-chars as these properties affect the sorption behavior of activated carbon the most (Ahmad et al., 2014). Godlewska et al.
(2019) indicated that higher temperatures resulted in better porosity and surface area in bio-chars. The BET surface area ranged from 65-150 m2/g. The surface area, porosity and polarity of bio-chars produced using N2 and CO2 were similar, but the aromatic nature changed with altering of the gas environment. Chars produced in CO2 showed better adsorbent behavior. Adding willow as a biomass did not impact the adsorbent properties of produced carbons.
Ros et al. (2006) also tested the impact of acid washing on the quality of activated carbons in term of surface area. They found that acid washing before activation increases the BET surface area significantly. In one experiment, the surface area increased from 7 m2/g to 269 m2/g.
Table 1: Summary of recent studies that used physical activation of sewage sludge.
Sludge type
Carbonization conditions Activation conditions SBET
m2/g
Reference T
(°C) t (h)
HR C/min
atm T
(oC)
t (h) HR C/min
atm
SSL 750 0.5 20 N2 34.3 Jindarom et al. (2007)
750 0.5 20 CO2 60.7
SSL 300 1 N2 850 0.67 Steam 130 Li et al. (2011)
Paper mill SSL
300 1 N2 850 0.67 Steam 280 Li et al. (2011)
Paper sludge
600 6 Air 22 Hojamberdiev et al.
(2008)
600 6 10 Steam 70
SSL 600 1 Steam 750 0.5 226 Rio et al. (2006)
SSL +willow biomass
700 3 N2 150 Godlewska et al.
(2019)
SSL 700 0.5 CO2 800 2-4 269 Ros et al. (2006)
SSL 800 4 10 CO2 97 Monsalvo et al. (2011)
SSL 400 4 10 Air 91
SSL 200 2 700 5 Djati Utomo et al.
(2013)
SSL 500 3 15 N2 Fan & Zhang (2008)
Dried SSL
650 0.5 40 N2 60 Rozada et al. (2008)
Husk, SSL
300 1 17 N2 51 Agrafioti et al. (2014)
SSL 500 0.5 17 N2 18 Méndez et al. (2005)
SSL 300 4 Agrafioti et al. (2013)
SSL 450 1.5 5 N2 60 Velghe et al. (2012)
SSL 400 2 20 N2 23.7 Zhang et al. (2013)
SSL 450 1 10 N2 275 4 10 Air 105 Méndez et al. (2005)
SSL 450 1 3 Inert Gascó et al. (2005)
SSL= Sewage sludge; HR= Heating rate; SBET= surface area determined by Brunauer, Emmett and Teller method, T =Temperature, t= time.