Abstract:
The objective of this research is to study the effects of sludge storage time on
capabilities ofactivated sludge (AS) to adsorb and desorb heavy metals. Zinc and lead were
utilized as studiedmetals. The activated sludges, ASI and ASII , were collected at different day
from filter pressprocess at Siphaya municipal wastewater treatment plant and used for adsorption
isotherm study.Organic matter and contents in ASI and ASII were 34 and 39 percent and aluminium
contents were 29.9 and 46.8 mg/g respectively. For desorption study, AS mixed soils were used as
the adsorbents with different ratios of sludge to soil of 0:1 (ASS0), 0.15:0.85 (ASS15), 0.30:0.70
(ASS30) and 1:1(ASS50). These 15 height-column adsorbents had been previously passed by leachate
spiked with 2 and 20 mg/L. of lead and zinc respectively for 5 months which results to lead content of 0.040,
0.058, 0.242 and 0.356 mg/g and zinc content of 0.275, 0.767, 2.343 and 3.411 mg/g in the top 5
cm layers of ASS0, ASS15, ASS30 and ASS50 respectively.
The data from adsorption study can be explained well by Freundlich isotherm. The adsorption
coefficients (Kd) determined at low equilibrium concentration where the Freundlich coefficient, n
equals to unit were used for comparing the capability of adsorption of AS after different storage
time. The higher Kd of ASII than ASI indicated the higher adsorption capability possible due to
more polymer adding in sludge ASII (aluminium content related to polymer). The Kd of lead
decreased from 46.3 to 8.7 and 2.7 L/g and zinc from 1.1 to 0.77 and 0.57 L/g similarly, the organic
matter contents decreased from 34 to 24.1 and 19.8 percent for ASI as the storage time increase
from 0 to 8 and 12 months respectively. However for the desorption study, the slightly amount of
heavy metals were observed even keep the AS-soil adsorbents as long as for one year. Following the
method by the Hazardous Substance Act, B.E. 2535(1992) of Ministry of Industry Thailand, these
adsorbents have not been classified as hazardous waste due to the low desorbed amount. Moreover,
the storage time of AS-soil adsorbent slightly effected to the desorption of the absorbed heavy
metal (lead and zinc) as well as the background heavy metal (chromium and aluminium).