Application of Vermifiltration for Domestic Sewage Treatment

Abstract

Climate change has led to water shortages in semi-arid regions. SDG 13 was advocates for wastewater reuse. Zimbabwe uses centralised conventional sewage treatment systems. Vermifiltration combines filtration process and earthworms in sewage water treatment. Vermifiltration is efficient, viable, requires less expertise and can be decentralised. Vermifiltration technique was used in treating domestic septic tank sewage water. Design parameters and efficiency were determined and characterised Vermifiltered water parameters were compared against the Environmental Management Agency Statutory Instrument 6 irrigation water standards. Vermifilter media contained gravel and composted soil with 20g Eseinia fetida earthworms per litre of soil. Treatments were septic tank raw water, vermifilter and control biofilter. A duplicate analysis was conducted. Hydraulic retention time was 1 hour 40 minutes and hydraulic loading rate 163l/m2/hour. Disposed wastewater did not meet required EMA standards. Both filters were effective in treating domestic sewage. There was a significant difference between untreated and treated wastewater. Vermifilter and the control, significantly (p < 0.01) treated pH, turbidity, total dissolved solids total suspended solids, biological oxygen demand, nitrates, phosphates and total coliforms properties. Vermifiltered water met EMA standards for irrigation and non-potable water uses. Phytoremediation can be incorporated in the designs to increase efficiency.

1. Introduction

Climate change has led to water shortages in arid and semi-arid regions. Sustainable development goal (SDG) 13 on climate change has been set to combat climate change by advocating for wastewater reuse. Zimbabwe depends on conventional sewage treatment systems, which are expensive, centralised, inefficient due to increasing population dynamics, high maintenance and need of high expertise. Countries such as Australia and China have adopted the vermifiltration technique, which combines the filtration process and earthworms in the treatment of sewage water. Vermifiltration is efficient, cost-effective, requires less expertise and can be decentralised. This study applied the vermifiltration technique on the treatment of domestic sewage water from a septic tank. The aim was to determine the design parameters of a vermifilter, characterising its efficiency in the removal of selected physico-chemical, microbiological parameters and comparing vermifiltered water against the Environmental Management Agency (EMA) irrigation water standards.

1.1 Wastewater management

Reclaimed water is a product of treated wastewater that includes industrial and domestic effluent [1]. Wastewater treatment is a technique for sewage water management. It involves the removal of pollutants in wastewater that are a threat to the environment. It involves chemical, biological and physical processes for the removal of the pollutants in water using infiltration systems, trickling filters, wastewater stabilisation ponds and septic tanks. One of the main products is reclaimed water and sludge that is deposited into the environment [2].

The use of conventional systems for the treatment of wastewater is costly, requires high maintenance, is centralised and has no resource recovery of reclaimed water. However, some countries have adopted vermifiltration as an alternative method for the treatment of wastewater [3].

1.2 Vermifiltration

Vermifiltration is a wastewater treatment method that incorporates the use of earthworms and the infiltration system [4]. The earthworms act as bio-filters and are capable of degrading, digesting and decomposing organic waste [5]. This is achieved through promoting growth of beneficial decomposing bacteria, biological stimulation, aeration and chemical degradation. Studies have shown the efficiency of vermifiltration in controlling pH, chemical oxygen demand (COD), total dissolved solids (TDSs), biological oxygen demand (BOD5), turbidity and chlorides (Cl) [4, 6]. It was first established at a University in Chile by Professor Jose Toha in 1992 [7]. Other countries are now using the treatment system, and it has been recommended for developing countries [4].

It is an efficient, inexpensive technique, non-labour intensive, requires low expertise, can be decentralised and is environmentally friendly. One of the products is reclaimed water that can be used for irrigation, landscaping, fire protection, flushing and vermicompost [1, 6]. Therefore, this study intended to recover the wastewater for non-potable water uses through the application of vermifiltration for domestic sewage treatment from septic tanks at Lupane State University (LSU).

LSU and Lupane Town do not have any wastewater treatment plant that enables the access of reclaimed water. Septic tanks are used for wastewater treatment, and thus, there is no use of either recycled or reclaimed water. The university uses approximately 30,000 litres of water per day from the hostels and the dining hall that goes to the septic tanks, and it is not reused. The use of septic tanks could also contribute to the pollution of underground water and contamination of the soil. The wastewater treatment plants currently used in Zimbabwe need high maintenance. With the growing population at the LSU campus, there will be decrease in the treatment efficiency of the septic tanks due to overload [8, 9].

1.3 Justification

Vermifiltration is a low-cost method that is eco-friendly, non-labour intensive, and it enables the treatment and reuse of wastewater [10]. LSU wastewater is disposed into the environment using septic tanks, thus increasing chances of pollution of the soil and underground water. Vermifiltration induces a decrease in COD, chlorides, TDS, TSS and pH in an efficient manner. The water is disinfected and clean enough to be used for farm irrigation and other non-potable uses. Some developed countries are using the system, and it has been highly recommended for developing countries since it is economically feasible and does not require a lot of maintenance and expertise. This study would therefore enable LSU to be able to reuse its wastewater for multiple tasks such as landscaping, construction, fire protection, irrigation of sport grounds and crops. This technique could be further utilised at household level, thus promoting decentralisation of wastewater treatment. The application of this low-cost sewage treatment system would go a long way in availing wastewater for irrigation and other non-potable water uses especially in the semi-arid areas.

1.4 Broad objective

The main objective was to apply and characterise a vermifiltration system for domestic sewage treatment at LSU.

1.4.1 Specific objectives

The specific objectives were to determine the design parameters of a vermifilter (hydraulic loading rate, hydraulic retention time, soil type and earthworm species); to characterise the efficiency of the vermifilter for the removal of the physicochemical and microbiological parameters (pH, BOD, turbidity, nitrates, phosphates, TDS, TSS and total coliforms) from the wastewater in the septic tank and to compare the physicochemical and microbiological parameters of the vermifiltered water against EMA standards.

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].

Parameter		Blue	Green	Yellow	Red
pH		6–9	5–6	4–5	0–4
		9–10	10–12	12–14
Turbidity	NTU	≤5	—	—	—
Chloride	Cl	≤250	≤300	≤400	≤500
Phosphate	P	≤0.5	≤1.5	≤3.0	≤5.0
Nitrate	N	≤10	≤20	≤30	≤50
Soluble Solids		≤25	≤50	≤100	≤150
Dissolved oxygen % sat		≥60	≥50	≥30	≥15
Total Dissolved Solids		≤500	≤1500	≤2000	≤3000
Faecal Coliforms/100 ml		≤1000	>1000	>1500	≥2000

Table 1.

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

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 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.

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].

3. Methodology

3.1 Site description

The study was carried out at Lupane State University (18.9300⁰S, 27.7593°E) in Zimbabwe. This is a semi-arid area with annual rainfall of 550 mm, high summer and low winter temperatures. The university depends on septic tanks for sewage treatment. The design experiment followed a duplicate analysis setup with three treatments: septic tank effluent raw water (RW), vermifilter (VF) and a control (CF). Biofilters were designed and set up (two vermifilters and the control filter). Twenty-litre plastic buckets measuring 27 cm x 29 cm x 32 cm were used as biofilter containers. Media consisted of four layers of 10–14 mm gravel, 4-8 mm gravel, river sand, black soil with Eisenia festida and 3 cm top layer. Layers were 4 cm, 4 cm, 10 cm, 20 cm and 3 cm thick, respectively. Twenty grams of earthworms per litre of black soil were used. The control biofilters did not have earthworms.

3.2 Measured parameters and method of measurement

Total dissolved solids, pH, TSS, turbidity, BOD after 5 days, nitrates and phosphates and total coliforms (TCs) were measured using by the oven drying method, an electronic pH meter, Lovibond portable turbidity meter, BOD5 dilution method; Beckman Conlter UV/VIS Spectrum and Multiple Tube Fermentation Technique.

3.3 Data analysis

Water quality parameter analysis was done before and after treatment. Genstat 14.0 was used for statistical analysis. The Shapiro–Wilk test (p < 0.05) was used to check for normality of data. One-way analysis of variance (ANOVA) at 5% level of significance was used to test for significant differences in related parameters per treatment. Results were compared against the EMA standards (SI6 of 2007 on Effluent and Solid Waste Disposal).

4. Findings and conclusions

Design hydraulic retention time was 1 hour 40 minutes, with a hydraulic loading rate of 163 l/m²/hour. The design parameters of the vermifilter are shown in Table 2.

The designed vermifilter and control biofilters were efficient in treating domestic sewage for the selected chemical parameters. Analysis of variance revealed that the vermifilter and control filter significantly (p < 0.001) treated the chemical properties of the domestic sewage water (Table 3).

All chemical parameters levels (pH, BOD, NO₃, PO₄ and chlorides) of treated sewage using the two filters were significantly different from the untreated sewage. However, no significant differences were observed on the chemical properties of sewage water treated using the two filters. The percentage range reduction in BOD on the VF was 68–97% whilst in the CF it was 76–98%, PO₄ in VF it was 90–99%, in the CF 95–98%. An increase was observed in nitrates in the VF of 96–98% and CF 62–82%.

4.1 Effect of vermifilter and control biofilter on the turbidity of the sewage water

The turbidity level of sewage water treated using the two filters was significantly different from untreated sewage. However, no significant differences were observed in turbidity of water treated using the two filters. The percentage reduction was higher in the VF at 83–99% and at the CF it was 77–97% (Figure 1).

Figure 1.

Turbidity of influent and effluent.

Bars shown using different letters were significantly different at 5% level of significance. The vertical bars shown at the top of each bar show the standard error of difference of means.

4.2 Effect of vermifilter and control biofilter on total dissolved solids in sewage water

The study reveals that the vermifilter and control biofilter significantly treated the TDS from the domestic sewage (Figure 2). The TDS level of treated sewage showed no significance difference in the VF and CF. The VF shows an efficiency of 44–90% and in the CF it was 70–93% (Figure 2).

Figure 2.

TDS of influent and effluent.

Bars shown using different letters were significantly different at 5% level of significance. The vertical bars shown at the top of each bar show the standard error of difference of means.

4.3 Effect of vermifilter and control biofilter on total suspended solids of sewage water

There was a significant difference in the level of TSS in the untreated sewage and the treated sewage from the two biofilters. The two biofilters had no significant difference in their operation of removing total suspended solids. The percentage reduction in TSS was higher in the VF at 48–97% and 24–97% in the CF (Figure 3).

Figure 3.

TSS in influent and effluents.

Bars shown using different letters were significantly different at 5% level of significance.

The vertical bars shown at the top of each bar show the standard error of difference of means.

4.4 Efficiency of vermifilter and control filter in treating total coliforms in domestic sewage

The vermifilter and control efficiently treat the available total coliforms in untreated sewage. The total coliforms in untreated sewage were significantly different from total coliforms from the two filters. The VF and CF, however, did not have any significant difference in their treatment porosity. The percentage reduction in the VF was 78–98% whilst in the CF it was 97–98% (Table 4).

Parameter	Property
Capacity of vermifilter	27 cm x 29 cm x 32 cm
First gravel layer	10–14 mm (4 cm thick)
Second gravel layer	4–8 mm (4 cm thick)
Third layer (sand)	2 mm (4 cm thick)
Forth layer (Black soil and E fetida earthworms)	20 cm
Top layer (free space)	3 cm
Hydraulic loading rate	163 L m^2 h^−1
Hydraulic retention time	1.4 L h^−1
Water discharge	2.1 ml s^−1

Table 2.

Parameters of the designed vermifilter.

Treatment	pH	BOD	NO3	PO4
Untreated sewage (RW)	7.71a	49.9a	9a	6.9a
Treated sewage using vermifilter (VF)	6.79b	6.8b	376b	0.38b
Treated sewage using control filter (CF)	6.52b	7.5b	256c	0.23b
Overall mean	7	21.4	214	2.47
F probability	<0.001	<0.001	<0.001	<0.001
LSD	0.44	8.47	105.4	0.72
% CV	5.1	32.2	40.1	23.7

Table 3.

Analysis of variance of the efficiency of the vermifilter and control biofilter in treating domestic sewage on the selected chemical parameters.

Treatment	TC
Untreated sewage (RW)	180a
Treated sewage using vermifilter (VF)	14b
Treated sewage using control filter (CF)	4b
Overall mean	66.1
F probability	<0.001
LSD	9.46
%CV	11.6

Table 4.

Analysis of variance on the efficiency in total coliform removal of a vermifilter and control filter in treating domestic sewage.

Septic tank effluent did not meet the required EMA SI6 Standards for disposal into the environment. Both filters were effective in treating domestic sewage. There was significant difference between untreated and treated wastewater on selected physico-chemical and microbiological parameters. Vermifilters and control significantly (p < 0.01) treated the physico-chemical (pH, turbidity, TDS, TSS, BOD, nitrates, phosphates) and microbiological (total coliforms) properties of domestic sewage water.

4.5 Comparison of effluent with EMA guidelines (SI 6 of 2007 on effluent and solid waste disposal)

Table 5 shows that the domestic sewage from the septic tank was not within the EMA standards for effluent disposal except for pH and nitrates.

Parameter	Raw Water	Vermifilter	Control Filter	EMA
pH	7.71	6.79	6.52	6─9
TDS	1230	261.3	179.5	≤500
TSS	259.7	46.9	54.1	≤25
Turbidity	207	12.79	14.99	≤5
BOD	49.87	6.79	7.47	≤30
Nitrates	9.01	275.8	250.8	≤10
Phosphates	6.80	0.38	0.23	≤0.5
Total Coliforms	180+	14	4	≤1000

Table 5.

Sewage quality parameters for disposal of the effluent relative to EMA standard guidelines.

The EMA Statutory Instrument 6 on Irrigation water standards uses the colour codes blue, green, yellow and red to indicate the level of pollution. Hence the colour shows whether the parameter is with the permissible limits or not.

4.6 Discussion

4.6.1 Change in chemical parameters of the wastewater

There was a decrease in the pH, phosphates and BOD after treatment in both the VF and CF. An increase occurred in the level of nitrates after treatment.

4.6.2 pH

The pH of the untreated domestic sewage was within the EMA standards. It is due to the decomposition of organic matter that occurs in the septic, thus reducing acidity and alkalinity of the domestic sewage. The treated sewage both from water from the CF and VF had a pH range that decreased though it was within the EMA standards. These results in the CF were due to the biological reactions that occurred through the infiltration process. The pH of the vermifilteted water drew closer to neutrality due to the vermicasts produced by the earthworms which are more neutral. Similar results were obtained by [4, 6].

4.6.3 Phosphates

Analysis of variance revealed that the biofilters were significantly efficient in treating the domestic sewage. The decrease in phosphates in the VF is attributed to the availability of anoxic conditions, biological metabolism and iron corrosion. The earthworms accumulate phosphate in the gut system, hence reduce the level of phosphates in the water.

4.6.4 Nitrates

The analysis of variance results showed that there was a significant difference in the untreated sewage and the treated sewage using the VF and CF. However, a significance difference also existed within the two filters. Generally, the nitrates in raw sewage are supposed to decrease during the treatment as they are supposed to be converted from nitrate and further decomposed to nitrogen gas and water through the nitrification and denitrification processes. The CF and VF had an increase in nitrates due the removal of ammonia through adsorption which leads to the formation of nitrates through biological nitrification. The earthworms also have a lot of nitrifying and denitrifying bacteria in the gut of worms.

Therefore, the presence of oxygen, aerobic Nitrobacter and the worm cast oxygenates the influent, thus leading to the increase of nitrates [34]. The nitrates formed are, however, made available to the plants. This is due to the secretion of polysaccharides, proteins and other nitrogenous compounds; these mineralise the nitrogen and make it available to the plants [35]. Similar results were obtained by [6, 9].

4.6.5 Bod

Vermifilter was significantly effective in reducing BOD which is attributed to the absorption of the organic waste by the worms through their body wall and the uptake through their gut. There is high effective symbiotic relationship between the earthworms and the soil microbes. They accelerate and increase the rate of decomposition of the organic matter [36, 37]. The muscular action of the foregut of the earthworms plays a crucial role. The organic matter is degraded, homogenised, conditioned, and biological activity in filter is improved. A decrease in BOD in the control filter is due to the filtration process and the action of bacteria that grow in the filter as well as the presence of the biofilm that is formed.

4.6.6 Decrease in physical parameters

There was a significant difference in all the selected physical parameters on the untreated and treated sewage after experimentation.

4.6.7 Turbidity

The analysis of variance results revealed that the vermifilter and the control filter were effective in treating domestic sewage. The decrease in turbidity is due to the adsorption of both the macroscopic and microscopic suspended solids in the filters. The adsorption occurs in the soil and gravel. The VF was more efficient due to the role of earthworms. The earthworms feed on the organic matter, thus reducing organic waste and proportionally controlling turbidity.

4.6.8 Total dissolved solids

The vermifilter and the control filter had a significant effect on the treatment of TDS in the domestic sewage water. The VF and CF water were within the EMA standards. The decrease of the TDS in the VF is attributed to the ingestion of the organic and inorganic solid particles in the RW by the earthworms. The earthworms excrete finer particles. The TDSs both in the CF and VF are trapped through adsorption and stabilised during the infiltration process [38].

4.6.9 Total suspended solids

There was a significance difference in the total suspended solids in untreated and treated sewage. A decrease in the CF is a result of the TSS sticking on the surfaces of individual filter media such as gravel and sand through adsorption and trapping of the suspended solids [39]. The removal of TSS in the VF was more effective as the TSS trapped on the filter reprocessed by the earthworms and fed to the microbes in the soil for further decomposition. Earthworms burrow within the soil and this increase aeration, thus enhancing efficient filtration and soil stabilisation [29].

4.6.10 Change in the total coliforms

The coliforms decreased in the VF due to the ability of earthworms to release coelomic fluids. These fluids have antibacterial properties and are capable of destroying all the pathogens in the biomass. The earthworms ingest the organic matter which they process through culling up the harmful microorganisms. The end products are deposits which are mixed with minerals and beneficial microbes free from pathogenic particles in the soil [39]. The decrease is also attributed to gizzard and intestinal enzymes of the earthworms. They are secreted through their body wall [40]. The decrease in the CF of total coliforms was due to the action of the biofilm that destroys the bacteria in the wastewater.

4.7 Conclusion

The water from the VF and CF was both within allowable limits of the SI 6 standards by EMA, except for the nitrates and turbidity. Total suspended solids were in the blue allowable range in the VF, whilst in the CF it was on the yellow range which is a threat to the environment. The vermifilter met the blue and yellow allowable limits that are even permissible for irrigation. The vermifiltered water can therefore be used for irrigation of parks and landscaping.

The vermifilter and the control filter were both effective in treating the domestic sewage from the septic tank. All the physicochemical and microbiological parameters were within the EMA regulation standards for disposal. They ranged between the blue-yellow range of acceptance. However, the most effective filter was the vermifilter.

The control filter is not sustainable since with time it clogs, discharge of effluent rate lowers and there was a foul smell. It is rich in nutrients that are nitrates and phosphates that are in an available form to the plants. There is a potential for the recycling of the sludge in the septic for use as manure and production of biogas.

The study shows that there is a potential for the recycling of the sludge in the septic for use as manure and production of biogas. It is recommended to carry out water quality assessments on borehole water and soil to see if there is any contamination as a result of the use of septic tanks.

Design improvement of the vermifilter can be done by incorporating phytoremediation as to increase treatment efficiency as to control the increase in nitrate concentration.

Acknowledgments

The authors appreciate the assistance given by Mr. R. Mudziwapasi for guidance in the experimental setup; Mr. E. Ndlovu for data analysis; L. Musaradenga, Mr. T. Kachote and Miss G. Dube, who assisted in laboratory analysis. Thanks to Mr. D. Sibanda and Mr. C. Moyo from the Works and Physical Planning Department for the provision of designing tools and their assistance.

Conflict of interest

The authors declare no conflict of interest.