The seedpods and lobs of the plant M. sylvestris were collected during spring from fields in Crete (Greece). The seedpods of the plant H. esculentus were bought from a local super market (originally grown in Northern Greece). In order to obtain mallow and okra mucilage, the same procedure was followed for both species. They were initially washed thoroughly with water to remove any impurities, dried at 110 °C for 6 h and then ground with a microhammer cutter mill and sieved to a 32 mesh (500 μm) particle size. Stock solutions of 50, 5 and 0.5 g/L were prepared and stored. The coagulation agent used was aluminum sulfate [Al2(SO3)4, extra pure, Merck].
The H. esculentus used in this study is the same species used in Agarwal et al. [15,16] and Mishra et al. [35], where the substances and properties of okra mucilage are discussed in detail. Regarding M. sylvestris, there are no reports of the species used as flocculant for the treatment of either drinking water or wastewater. M. sylvestris is botanically related to H. esculentus, therefore it may also show similar flocculation behavior.
The flocculation effect of mallow and okra mucilage was studied in synthetic wastewater and biologically treated effluent. The synthetic wastewater was prepared by adding 100 mg of kaolin (powder, Ph. Eur. standard, Sigma-Aldrich) and 10 mg of humic acid (technical grade, Sigma-Aldrich) in 1 L of distilled water. The suspension was then stirred at 100 rpm for 2 h. Κaolin was added to increase turbidity, whereas humic acid was added to maintain the carbon concentration proportional to the biologically-treated effluent. Biologically treated effluent (from extended aeration and secondary sedimentation) was obtained from the wastewater treatment unit of the Technical University of Crete.
Turbidity was measured with a Lovibond Turbidity Meter using ASTM method D7315-07a (Standard Test Method for Determination of Turbidity above 1 Turbidity Unit (TU) in Static Mode). The turbidity removal efficiency of the mucilages was calculated using the following equation: