**2. Review of literature**

## **2.1 Wastewater management**

Wastewater management is the process of managing effluent through treatment, safe disposal in the environment and for reuse [11]. Wastewater consists of industrial, domestic and agricultural effluent. Wastewater contaminants include plant nutrients, pathogenic micro-organisms, heavy metals, organic pollutants, biodegradable organics and micro pollutants [6]. Wastewater management facilitates complete removal of pollutants for environmental protection, human and animal health [12]. The World Health Organisation (WHO) Guidelines [13] and SDGs advocate for efficient wastewater treatment and management to cater for future projected water scarcity, safe drinking water, sanitation and sustainable environmental management [14].

Wastewater mismanagement has detrimental effects on the environment, people, animals and aquatic life. Water pollutants disposed into the environment promote water pollution [15]. Wastewater is a drought-resistant resource in households,

industry and agriculture if the sludge and water can be treated and reused [16]. Wastewater can be reused as a nutrient source in agriculture, irrigation, energy (biogas) and soil conditioning [4]. Efficient wastewater treatment promotes prominent levels of sanitation and sustainable development within nations.

### **2.2 Sewage treatment**

Biological, physical and chemical techniques are used in wastewater treatment [11]. These include natural water purification processes that occur in oceans, lakes and streams. Treatment systems can be centralised or decentralised depending on the economy, technological advancement and population [17]. Zimbabwe's treatment systems include infiltration, waste stabilisation ponds, trickling filters, activated sludge systems, septic tanks and pit latrines [8].

### **2.3 Wastewater management in Zimbabwe**

Wastewater management in Zimbabwe is governed through legislation and policed by the Environmental Management Agency. There are regulations and policies that stress on the management of wastewater. These include the Environmental Management Act CAP 20:27) and other regulations. The regulations are the Statutory Instrument 6 of 2007 on Effluent and Solid Waste Disposal, Public Health Act (CAP 15:09), Urban Councils Act (CAP 29:150, Municipal bye-laws and Rural District Councils Act (CAP 29:1) [1]. These acts and bye-laws help in the control and monitoring of efficient treatment and disposal of wastewater. In most urban areas, the treatment of wastewater is overseen by the town municipality. The aim is meeting the standards for disposal and reuse of the reclaimed water for other non-potable uses.

### *2.3.1 Sewage treatment plants used*

Wastewater management consists of various means for the treatment of the wastewater. The treatment methods include biological, physical and chemical techniques [12]. These systems incorporate the natural processes that occur naturally in oceans, lakes and streams in water purification. The treatment systems can either be centralised or decentralised depending on the economy of the country, technological advancement and its population [17]. The treatment plants vary in preference in terms of land requirements, sludge production, efficiency, reliability, affordability and energy consumption.

The treatment systems for wastewater used in Zimbabwe include the infiltration systems, waste stabilisation ponds, trickling filters, activated sludge systems, septic tanks and pit latrines [8, 18]. These systems treat both industrial and domestic effluents; however, some are limited to domestic effluent such as the waste stabilisation ponds and pit latrines. These conventional systems for wastewater treatment are mostly centralised where the municipality controls and monitors the treatment processes. In urban areas, there are sewerage pipelines from households and industries that are directed to the designated treatment plants. In industries, it is a must for the effluent to be firstly pre-treated before being directed to the municipality pipeline. This is to protect the sewerage pipelines and the treatment plants from being corroded and clogged by the toxic effluent from the industries.

The conventional system plants used require heavy maintenance and are faced with tremendous pressure due to high populations [19]. The treatment plants are

therefore viewed inefficient in the treatment. This is supported by the pollution of water courses such as Lake Chivero in Harare and Umguza River in Bulawayo. These have been invaded by *Eichhornia crassipes* (water hyacinth) due to pollution caused by the inadequately treated effluent. Another recent cause was of the outbreak of cholera which was due to the mixing of drinking water with sewage effluent. This shows the inefficiency of the wastewater management in Zimbabwe [20].

### *2.3.2 Parameters monitored*

The treatment plants are set to meet certain standards in the treatment process for the disposal and reuse of the waste water [1]. These standards are set against EMA standards such as SI6 of 2007, SAZ and WHO standards. The Statutory Instrument 6 of 2007 on Effluent and Solid Waste disposal is the one mainly used in Zimbabwe. **Table 1** shows the categorised limits using colours for disposal. The parameters monitored include physical, chemical and microbiological pollutants. These include pH, turbidity, suspended solids, dissolved solids, dissolved oxygen, nitrogen, phosphates, ammonia, alkalinity, BOD, COD, coliforms and metals. The treated effluent should meet the set safe standards for disposal and reuse. The treatment plants are designed to reduce and control the pollutants found within the effluent [19].

**Table 1** shows Statutory Instrument (SI) 6 of 2007 on Effluent and Solid Waste Disposal in Zimbabwe.

Generally, the treatment plants in the country are expensive to maintain. Most of them in urban areas are centralised, hence the loading density becomes very high to maintain optimum treatment. This leads to the recurring sewer bursts perpetuating water pollution [18]. The lack of the proper management of sludge is another problem which finds its way in the landfills contributing to the production of methane [21]. There is no resource recovery in some treatment plants such as the septic tanks.


*Key: Blue = Good-quality effluent suitable for disposal to the environment.*

*Green = Satisfactory quality effluent.*

*Yellow = Poor quality effluent not suitable for disposal to the environment.*

*Red = Very-poor-quality effluent attracting heavy disposal fees.*

### **Table 1.**

*Environmental management agency (effluent and solid waste disposal) regulations SI 6 of 2007).*

Decentralisation and resource recovery of the wastewater from the treatment plants can be the solution to the current situation of Zimbabwe. With the growing populations and water scarcity due to climate change, alternatives to cater for the challenges have to be put into place. Decentralisation and resource recovery can be utilised by everyone and capitalised at household level [22]. Other countries are using the vermifiltration treatment system which enables resource recovery and decentralisation of wastewater treatment, and it has been recommended for adoption by developing countries.

## **2.4 Vermifiltration**

Vermifiltration is an infiltration method combined with the biological means of treating wastewater. The main actors in the vermifiltration process are earthworms. The earthworms act as the biofilters that destroy the waste. Studies on vermifiltration for domestic and industrial effluent have been carried out in other countries, and its efficiency has been proven [5, 10]. It was first established at a University in Chile by Professor Jose Toha in 1992 [7]. Some studies on vermifiltration that have been that done showed efficient removal of BOD, COD, suspended solids, total dissolved solids in wastewater of 80–90% removal [4, 6, 10]. A few studies on vermifiltration for the treatment of domestic effluent have been done in Zimbabwe. The studies showed a percentage reduction of above 70% of pH, BOD, COD, TDSS and turbidity [4, 9]. The use of vermifiltration proved to be more efficient and environmentally friendly as compared with the conventional systems used. However, less has been done on implementing it on treatment plants such as septic tanks that have no recovery of the wastewater.

### *2.4.1 Treatment process in vermifiltration*

The treatment process in vermifiltration involves the vermicomposting process and the microbial processes in the removal waste loadings in the effluent. The vermifiltration media is usually characterised of granular materials which could be gravel, quartz of layers of various sizes, ceramsite and soil which is inoculated with earthworms [23]. The granular materials help in the removal of pollutants through filtration. The earthworms play the role of degrading the waste and activating the growth of microbiological organisms that decompose the waste [24, 25].

### *2.4.2 Earth worm action*

There are various earthworm species that can be used for vermifiltration, since they are well known as the waste and environment managers [26, 27] The earthworms are long, narrow, bilateral, cylindrical segmented species with no bone formation. They have millions of nitrogen fixing and decomposing microbes in their guts. They have chemoreceptors that assist them for the searching of food. The types of earthworms that are effective in wastewater treatment include the African Night Crawler *(Eudrillas eugenor*), red tiger *(E. andre*), Indiana blue worm (*Feihoye excavatoxa*) and tiger worm *(Eisenia fetida)*. These earthworms have proven to be efficient; however, most studies have shown that *E. fetida* works more efficiently compared with the other earthworm species.

*E. fetida* worms are epigeic versatile waste eating worms with digestive enzymes such as protease, alkaline, phosphates and cellulose and use their bodies as bio filters. They feed on organic waste in wastewater; promote growth of decomposing bacteria and increase aeration and biological stimulation in the bioreactor. They have a very wide temperature tolerance of 20–25°C and can live in organic waste with a range of 60–75% moisture content. They prefer dark moist soil and can tolerate pH as low as 4. In their action, the earthworms excrete microbial organisms which consist of nutrients such as phosphates and nitrates that are further utilised by the micro-organisms for the decomposition of the waste [4, 6]. The earthworms also feed on the sludge from the effluent and degrade suspended solids that are trapped on top of the filter and fed on the soil microbes. Adsorption and stabilisation of inorganic solids, dissolved and suspended solids occur through biodegradation within the media inhabited by earthworms.

The burrowing earthworms increase aeration processes; thus, it enhances the filtration and soil stabilisation to be more efficient. Choking and production of foul smells are prevented [6, 28]. The earthworms generally act as the biofilters in the system by adequately treating the effluent [29].

### *2.4.3 Hydraulic retention time (HRT), hydraulic loading rate (HLR) and flowrate*

The hydraulic loading rate (HLR) is the volume of wastewater applied per unit area of the bed per unit time. It is influenced by the volumetric flow rate, number of live adult earthworms functioning per unit area and their health. On the other hand, the hydraulic retention time (HRT) is the time taken by the wastewater in interaction with the worms. It depends on the flow rate of the wastewater, porosity of the soil and the volume of the soil profile. The higher the retention time, the more efficient the treatment system becomes. The HRT is also influenced by HLR; the higher the HLR, the lower the retention time, thus a decrease in treatment efficiency occurs.

### **2.5 Resource recovery from vermifiltered wastewater**

Vermifiltration leads to treatment of wastewater that can be reused and vermicompost is formed as a by-product. The treatment of wastewater by vermifiltration has proven to be more effective as in some studies, it has achieved approximately more than 90% removal of some pollutants. This makes the water reusable for non-portable uses [7]. With the increasing demand and shortage for water, the reuse of wastewater is an alternative for combating water scarcity and the protection of the available water sources.

The vermifiltered water is clean enough and contains some nutritive nutrients that could be of use in agriculture. Irrigation can be carried out using the reclaimed water. In drought seasons, irrigation can also be practised continuously with the use of reclaimed water [22]. The water will be containing nutrients such as nitrates which increase the fertility of the soil. The plants will not be attacked by water nor nutritive stress. Where there are treatment plants with no reuse of wastewater, vermifiltration can be implemented and developmental projects of irrigation will emerge. The reclaimed water can also be for fire protection as compared with the use of fire hydrants and fire extinguishers. This again aids in water preservation and the use of

environmentally friendly techniques. This can be done at household level through the decentralisation of wastewater treatment by the introduction of vermifiltration. Fire awareness and preparedness will therefore be improved and implemented at large scale.

The reclaimed water can be used for domestic use for the flushing system. This reduces the pressure on the freshwater resources used. The loading in the treatment plants and sewage pipes will be reduced, thus preventing the occurrence of sewage bursts. This will be due to the reuse of the wastewater for flushing system. This will also control the occurrence of diseases since during load shedding, people tend to use undesignated areas for relief, thus perpetuating disease emanation. The use of reclaimed water thus can act as a substitute and alternative for the use of freshwater for the flushing system [30].

The reclaimed water and the vermicasts from vermifiltration act as fertiliser in agriculture [31]. This water can be used for landscaping in areas which are dry. Some areas which have water shortages and infertile soils can have the opportunity of practising landscaping. The wastewater can be reused for the watering of ornamental flowers and introduce loans. The infertile soils will be made arable by the treated effluent. The maintenance of the landscape will be much easier as the production of the reclaimed water is throughout the year [32, 33].
