Aare Kuusik and  E. Loigu and  O. Sokk and  Argo Kuusik,  Enhancement of Methane Productivity of Anaerobic Reactors of Wastewater Treatment Plants.   journal   = {International Journal of Environmental and Ecological Engineering}, [online]. World Academy of Science, Engineering and Technology.
    July 2012, vol. 67(7). 460 - 462
    [viewed 16 October 2019]. Available from: https://publications.waset.org/pdf/9223.