Municipal wastewater is characterized by its physical attributes as well as its organic and inorganic contaminants. Physical attributes include color, odor and turbidity caused by dissolved or suspended solids. Organic contaminants include dissolved volatile organic com-pounds (VOCs), which include phenols, chlorobenzene, hydro-carbons and dissolved or non-dissolved non-volatile organic compounds designated as chemical oxygen demand (COD). Inorganic contaminants may include compounds of trace metals such as copper, zinc and lead.
Production and processing of pecans in Louisiana is estimated to generate nearly 6 million poundsof shells annually. Although some pecan shells have found limited applications in landscape mulch, most are discarded, creating a significant environmental problem.
LSU AgCenter research has demonstrated that pecan shells can be converted to granular activated carbons, which can be used to effectively treat contaminated water such as municipal wastewater and can be manufactured at a compet-itive price of $1.20 to $1.40 per pound compared to more than $2 per pound for some coal-based commercial carbons, which are widely used in water purification.
The process of adsorption as practiced by activated carbons involves attraction of the molecules to be adsorbed to the inner surface of the carbon pores. Since activated carbons have considerable surface area, they can attract a significant quantity of adsorbing material.
The advantage of using agricultural by-products as raw materials for manufacturing activated carbon is that these raw materials are renewable and potentially less expensive to manufacture. Furthermore, the use of Louisiana’s pecan shells as feedstock for carbon pro-duction would add value to the shells, turn the shells into value-added commodities and reduce waste disposal costs
for the shellers.
Commercial granular activated carbons, which are mostly produced from bituminous coal, are extensively used as filter beds in treatment of municipal wastewater. The wastewater flows through these filter beds, and organic and some inor-ganic pollutants are adsorbed on the surface of the carbon. When the carbon becomes loaded with the adsorbed molecules and loses its adsorptive ability, it is removed from the filtra-tion beds and regenerated. The advantages of using activated carbons in treatment of wastewater as opposed to conventional biological treatment include less land used in the treatment process, a higher level of treatment efficiency, reduced sensitivity to daily flow variations, and greater flexibility in design and operation of the treatment facility.
The objective of this research was to compare the effec-tiveness of pecan shell-based granular activated carbons, activated under different activation conditions, to a commonly used coal-based, commercial granular activated carbon (F200) for the removal of various contaminants commonly found in municipal wastewater. The activated carbons were pecan shells activated with phosphoric acid (PSA), pecan shells activated with carbon dioxide (PSC) and pecan shells activ-ated with steam (PSS). The four carbons were compared for removal of inorganic contaminants (lead, copper, zinc), volatile organic compounds (VOCs) (compounds of health concern and known toxic compounds, such as bromodi-chloromethane, benzene, carbon tetrachloride, 1,1,1-trichloroethane, chloroform, and 1,1-dichloroethane) from a laboratory prepared solution, and COD (chemical oxygen demand) from a municipal wastewater.
The adsorption studies on a multi-component metal mixture consisting of copper, lead and zinc ions (Figure 1) indicated that PSA had the highest adsorption efficiency for zinc and copper ions followed by PSS when compared to commercial carbon. PSC had the lowest adsorption efficiency among the carbons studied.
Results from Table 1 indicate that PSS was generally better than the other experimental carbons and the commercial carbon in adsorption of select VOCs, inferring that the method of activation and precursors selected for the preparation of activated carbons affect the adsorption of VOCs and hence
are factors to be considered in any adsorption process. All the carbons studied exhibited efficient adsorption of benzene. Acid-activated experimental carbon (PSA) showed higher adsorption of 1,1,1-trichloroethane compared to all the other carbons.
Primary treated municipal wastewater was collected from the Central Municipal Processing Plant at Baton Rouge, La., for removal of organic contaminants measured as COD. The results from Table 2 indicate that steam-activated and acid-activated pecan shell-based carbons had higher adsorption for COD organics than carbon dioxide-activated pecan shell-based carbon or coal-based carbon at the carbon dosage used during the experiment. The higher adsorption may be related to surface area because the two carbons with the highest surface area (PSA and PSS) also had the highest COD adsorp-tion. These results show that granular activated carbons made from agricultural waste (pecan shells) can be used with greater effectiveness for COD removal from municipal wastewater than a coal-based commercial carbon.
Results show promise
Based on the overall results of the study concerning organic (COD and VOCs) and inorganic metals, it is suggest-ed that experimental carbons could be a viable and cost-effective option compared to commercial carbons in munici- pal wastewater treatment systems. Future plans include testing the experimental carbons in real-world situations in municipal wastewater treatment plants followed by commercialization of the product. Several rural and small city municipalities, pecan shellers, as well as entrepreneurs from within Louisiana and adjacent states have expressed interest and offered support and cooperation.
(This article appeared in the spring 2004 issue of Louisiana Agriculture.)