Investigating the application of constructed wetlands in septic tank systems built on clay soils. A case study of Clydesdale Park, Gweru.

The study is an assessment of the behaviour of clay soil where septic tanks are built on, to evaluate the hydraulic performance of a septic tank system with a constructed wetland and to evaluate an economic feasibility of constructed wetlands in septic tank system built on clay soil. As a result, due to low drainage of clay soils, wastewater does not drain easily in the clay soil matrix. This is undesirable as it exposes the community to waterborne diseases. The study therefore investigated the applicability of constructed wetlands in septic tank systems built on clay soils. The scope behind the study was to see if planting wetland vegetation on the soak away area can assist to dispose of the excessive wastewater through plants’ high evapo-transpiration rates. The results showed that wetland vegetation plants such as common reeds (Phragmites australis) and cattails (Typha latifolia, C) had up to 70% rate of evapo-transpiration. This rate of transpiration can dispose safely wastewater flow from households. Furthermore, this wetland vegetation can serve as a habitat for animals like birds, frogs, and can act as a local “green space”. With this practicability, this wetland vegetation can effectively solve wastewater disposal challenges faced in septic tanks built in problematic soils of poor drainage. Since the septic tank systems constructed on wetland vegetation principle rely on evapo-transpiration, they require a larger surface area on which the vegetation will be planted.

According to the design of the study, wetland vegetation coverage of twenty two square metres can suffice the efficient operation of this system which can be provided in two wetland soak ways of eleven square metres. The results showed that septic tanks constructed on the principle of wetland vegetation are slightly expensive but economically comparable to the conventional ones of soak ways. The study recommends for the practical construction and setup of this wetland vegetation to further establish its applicability.

Materials and Methodology

Literature survey was carried out on municipal by-laws and policies to determine the spaces and distances required for the positioning of the modified septic tank system from boundaries of the stand and the house. The size and surface area of the constructed wetland and the soak away were obtained from modelling the designs of the two systems. Plants commonly used in wetlands were found in literature. The desired system did not have open water bodies. The area to be covered by the constructed wetland was calculated against that of a soak away to determine if the system is feasible at a low-density stand. The plants were elected according to their adaptability for growth under partial conditions. The choice of the type of constructed wetland is dependent on human health and environmental protection for the system. Sub-surface wastewater flow was used to avoid breeding spaces for disease causing organisms and unpleasant odours. The data of the wastewater produced from each household per day was obtained by considering the water demand of the household. The daily water demand was multiplied by the percentage (50%), to get the wastewater generated. A price survey on capital costs of constructing a soak way and a constructed wetland were carried out. The data of the space area and retention time were obtained from the modelling of the modified system. The operation and maintenance costs were determined by costs of equipment, power, labour and spare parts.

Results and Analyses

From the study the common reed plant and cattails had up to 70% rate of evapotranspiration. Common reeds and cattails also are aesthetically pleasing and add greenery to a built-up area. They serve as a habitat for animals such as birds and frogs, and also act as a green space within. The plants are commonly used due to their climatic tolerance and rapid growth. Subsurface flow constructed wetlands can even be walked on and they avoids mosquito and odour problems. From literature the design water demand per stand is 1800litres per day, BOD5 per user is 333mg/l, low-density capita per house size is 6 people as according to the design approach to water and sewerage relative to urban and rural communities in Zimbabwe manual. In Gweru,
• On average, the warmest months are January, October and November
• On average the coolest month is July
• December is the wettest month
• August is the driest month
The economic feasibility evaluation of constructing a modified septic tank system was done based on the cost of capital of building the system. To achieve this as a comparison of the original and modified septic tank system costs was done by the use of bill of quantities. There is significant difference between estimated project costs of the constructed wetland as compared with the soak way indicating how economic it is to use constructed wetland as a disposal system on onsite sanitation.

Conclusion and Recommendations

Constructed wetlands are environmentally friendly and feasible. The replacement of a soak way with a constructed wetland as a disposal unit proved to be feasible with constructed wetland disposing of through evapo-transpiration. Constructing two wetlands in series increases the evapo-transpiration hence enhancing the disposal capacity of the constructed wetland. A cost-benefit analyses between the two systems showed that constructed wetlands requires an additional amount of money on top of that of the soak away although constructed wetland is more beneficial in areas where clay soil exists. This solution of replacing soak away with constructed wetlands can be applied when the soil in the area of interest is clay. The researcher recommended for a full economic evaluation of constructed wetland to be performed which include operation and maintenance costs.

Should you wish to access more information than is provided, please contact Us