The authors regret that the following information was not correctly shown in the acknowledgements in their original paper, and apologise for any inconvenience caused:

This paper was made possible with UK Aid from the Department of International Development (DFID) as part of the Sanitation and Hygiene Applied Research for Equity (SHARE) Research Consortium (http://www.shareresearch.org). However, the views expressed do not necessarily reflect the Department's official policies.

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