Wind Speed Thresholds for Adjusting Conventional Design Criteria for the Ventilated Improved Pit Latrine without Compromising Odour Control

Abstract

Aims: The objective of this paper is to re-evaluate the wind speed threshold at which the 100 mm polyvinyl chloride (PVC) vent pipe may be used without compromising the ventilation rate required for effective odor control in a ventilated improved pit (VIP) latrine. The paper also sought to investigate the chances of leveraging upon high wind speeds to grant user preferences for multiple windows and installation of insect screens, which are conventionally forbidden. Study Design: A comparative experimental design was carried out in a VIP latrine with internal cubicle dimensions of 1.2 m x 1.5 m built over a pit of cross-section 1.2 m x 2.5 m and depth of 3.0 m which was fitted with a 100 mm PVC vent pipe. Place and Duration of Study: The experimental setup was located on the compound of a public basic school in Prampram, a peri-urban coastal community in Southern Ghana. The setup was monitored for twelve hours a day (5 am to 5 pm) for twelve days. Methodology: The experimental setups comprised of four designs of the VIP latrine: single-window and multi-window designs each with and without insect screen in the window. The ventilation rate in each modified design was monitored simultaneously with the external wind speed. Results: The results of the study indicate that the 100 mm PVC vent pipe could attain the recommended ventilation rate of 20 m3/h at an average wind speed as slow as 1.5 m/s as compared to the 3 m/s previously suggested.  The influence of wind speed on the ventilation rate is insignificant at wind speed variations not exceeding 0.5 m/s when the 100 mm PVC vent pipe is used. Conclusion: At an average wind speed of 1.5 m/s, either the multiple-window design or insect screen used in combination with the 100 mm PVC vent pipe could attain the recommended ventilation rate and should not be outrightly forbidden. However, a wind speed threshold of 2.5 m/s is required for the adoption of the multi-window design with insect screens when the 100 mm PVC vent pipe is used.