Garden cress mucilage as a potential emerging biopolymer for improving turbidity removal in water treatment

Highlights

• Garden cress (Lepidium Sativum sp.) biopolymer exhibited anionic nature.

• Presence of hydroxyl, carboxyl and methoxy groups in the biopolymer promoted linking and binding mechanism in flocculation.

• Addition of biopolymer produced a more compact sludge and achieved 92% turbidity removal in 1 min.

Abstract

Agricultural anthropogenic processes have resulted in high turbidity to nearby river water. This study aims to study the possibility and mechanism of biopolymer extracted from garden cress (Lepidium Sativum sp.) seed as natural coagulant aid in a point source agriculture wastewater discharge. Various physico-chemical characterisation studies were conducted i.e. functional group, zeta potential and surface floc morphology. FTIR spectra results indicated the biopolymer was assigned to band peaks at for hydroxyl, carboxyl groups. Whilst, zeta potential was around −16 mV, describing its anionic nature. The morphologies study depicted that the addition of biopolymer produced a more compact and larger structure of the settled sludge. Influence of pH, dosage of Fe³⁺ and biopolymer were studied in synthetic kaolin and agricultural wastewater using factorial design. pH, dosage of Fe³⁺ and biopolymer significantly affect the turbidity removal in coagulation process with p-value < 0.05. While, optimized setting is at pH 5, concentration of Fe³⁺ is at 50 mg/L and concentration of biopolymer is at 15 mg/L for turbidity removal of 99.32%. Interestingly, it could achieve more than 92% turbidity reduction within one-minute settling time at pH 6, concentration of Fe³⁺ is at 8–10 mg/L and concentration of biopolymer is at 2.8–5.0 mg/L.

Introduction

Over the century, scarcity of water and food has become apparent with the increasing population at an accelerating pace (Wyman, 2013). Earth will be occupied with 9.8 billion people by 2050 (Jones, 2017) and this raises the concern on depletion of natural resources due to overpopulation. One of the key factors leading to water quality deterioration and depletion is due to agricultural activities (Abraham et al., 2016; Saimy and Yusof, 2013). Irrigated agricultural processes account for about 85% of global water usage and 70% of global water withdrawals (Brauman et al., 2014).

In Malaysia, agriculture sector utilizes around 76% of surface water for irrigation purpose (Ahmed et al., 2014). With the growing population in Malaysia, increasing pressure on the water supply sources to agricultural development is inevitable to ensure the food security to the community (Ahmed et al., 2014; Saimy and Yusof, 2013). This requires expansion of irrigation and substantial increase of the usage of fertilisers and pesticides to yield higher production (Aregay and Minjuan, 2012; Wang and Xing, 2016). As a result, increasing amounts of wastewater which are high in turbidity, heavy metals, pesticides and herbicides further challenge the complexity of the treatment for water recovery (Amosa et al., 2016; Rott et al., 2017; Wang and Chen, 2015).

In view of this, the removal of these pollutants from agriculture using effective wastewater treatment techniques becomes great interest to cater for the growing demand of clean water. Most commonly wastewater treatments include chemical precipitation (Huang et al., 2017), biological treatment (Min et al., 2018; Song et al., 2018, 2017), adsorption (Wei et al., 2017), reverse osmosis (Epsztein et al., 2015), ion exchange (Wang et al., 2017), coagulation-flocculation (Ishak et al., 2018), advanced oxidation process (AOP) (Cai et al., 2018; Luo et al., 2018a; Luo et al., 2017; Luo et al., 2018b, Luo et al., 2018c) and electrocoagulation (Chen et al., 2018). Among all aforementioned approaches, coagulation-flocculation is simple yet economically viable process as it can be applied directly without toxicity disturbance to remove suspended particles, certain soluble compounds and very fine solid colloids that present in the wastewater by destabilising and developing flocs (Kumar et al., 2017; Lee et al., 2014b). This process destabilises the colloidal pollutants with coagulants and agglomerates them using flocculants. However, some of the coagulants such as aluminum sulphate (alum) poses menace to human health in association with Alzheimer disease (Salehizadeh et al., 2018). Besides, it also creates high volume of sludge residual. In Malaysia, two million tons of aluminum-based sludge from 462 water treatment facilities are produced annually with the use of alum in the water treatment (Breesem et al., 2014; Choy et al., 2016; Odimegwu et al., 2016). Furthermore, the sludge is classified as Scheduled Waste (SW204) and additional annual costs up to 6 million are incurred for sludge disposal only (Choy et al., 2016).

To ease this issue, several studies have demonstrated the use of natural coagulants such as moringa (Moringa oleifera Lam), Chickpea (Cicer arietinum Linn), chitosan and lablan bean (Dolichos lablab Linn) are effective in organic matter removal (Abebe et al., 2016; Choy et al., 2014; Ronke et al., 2016). On the other hand, polysaccharide bio-based flocculants such as alginate, cellulose and starch also received considerable attentions in wastewater treatment due to their wide availability, biodegradability and outstanding molecular structures (Salehizadeh et al., 2018). As compared with traditional synthetic flocculants, natural flocculants are harmless to human health and aquatic ecosystem as they are free from toxicity, non-corrosive and biodegradable in nature (Freitas et al., 2018; Wu et al., 2017). These novel properties make them reliable to be utilised in primary treatment process without the concerns of health risks to the microorganisms in secondary biological treatment (Freitas et al., 2018). Besides, they are good in structural integrity, reproducible from agricultural sources and generate no secondary contamination (Lee et al., 2014b).They possess specific molecular compositions with effective functional groups such as carboxyl and hydroxyl groups to neutralise and link the colloidal particles together (Lee et al., 2014b). The flocs formed will be larger and stronger which enhance the settling of the pollutants from stable suspension (Lee et al., 2014a).

Hence, the objective of this study is to evaluate the feasibility and effectiveness of biopolymer extracted from garden cress seeds as natural coagulant aid for turbidity removal in synthetic kaolin suspension and agricultural wastewater.