Abstract
The chemical basis for improved removal rates of toxic heavy metals such as Zn and Cu from wastewater secondary sludge has been demonstrated in this study. Instead of using excess corrosive chemicals as the source of free nitrous acid (FNA) for improved solubility of heavy metals in the sludge (in order to enhance electrokinetics), an optimized use of aqua regia has been proposed as an alternative. Fragments of nitrocyl group originated from aqua regia are responsible for the disruption of biogenic mixed liquor volatile suspended solids (MLVSS) and this disruption resulted in enhanced removal of exposed and oxidized metal ions. A diversity of nitric oxide (NO), peroxy nitrous acid, and peroxy nitroso group are expected to be introduced in the mixed liquor by the aqua regia for enhanced electrochemical treatment. The effects of pectin as a post treatment on the Zn removal from sludge were also presented for the first time. Results revealed 63.6% Cu and 93.7% Zn removal efficiencies, as compared to 49% Cu and 74% Zn removal efficiencies reported in a recent study. Also, 93.3% reduction of time-to-filter (TTF), and 95 mL/g of sludge volume index (SVI) were reported. The total operating cost obtained was USD 1.972/wet ton.
Highlights
Remove heavy metals[11,14], break down recalcitrant colloids and organics[15], reduce soluble microbial products[16], and enhance sludge physiochemical properties[6,17]
In EK, heavy metals are either removed by (a) flocculation and precipitation induced by the formation of coagulants through the hydrolysis of released anodic ions under the influence of electric field[18,19], (b) migrated toward the oppositely charged electrodes where they are removed via electrophoresis and electrodeposition[20], or (c) removed via floatation enabled by gas bubbles produced from or added to the process[21]
EK faces the following three constraints: (a) utilizing high electrical energy for optimum release of electro-coagulants may result in high electrical energy consumption and operation cost[18]; (b) biosolids do not have high electrical conductivity (EC) required to disperse and carry forward electric charges during EK treatment[22]; and (c) heavy metal removal efficiency in EK process is aided by low sludge pH which enhances the solubility of metals
Summary
Remove heavy metals[11,14], break down recalcitrant colloids and organics[15], reduce soluble microbial products[16], and enhance sludge physiochemical properties[6,17]. Among these, bioleaching - a process where sulfide form of metal is oxidized to sulfuric acid through microbial activity - has been found useful in decreasing the pH and solubilizing the metal[25] Another strategy is to apply an optimized efficient electrokinetic treatment followed by post treatment via organic acids or chelating agents to remove heavy metals from sewage sludge. These agents offer good coordination ability to the metal ions at wide pH range. In order to further improve the overall quality of the sludge, a biogenic chelating agent has been used to post-treat the sludge after the low strength acidified EK process This agent is a cation-capturing organic extract from citrus fruit peel waste known as pectin. An effective anode area of 86.88 cm[2] was used and a distance of 1.5 cm was maintained between the reactor bottom and electrodes for thorough mixing
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