Heritage Cases

The capacity of the Darvill Wastewater Works currently comprises 65ML/d. The increase in the hydraulic load, and associated organic load, has put a strain on the capacity of the plant to treat and effectively remove nutrients. Furthermore, the plant’s discharge limit for NH3 has been decreased to 6mg/l. As a result of the above, the plant is currently unable to meet required discharge standards, and needs to be upgraded. Changes to the Head of Works, the biological process, sludge digestion, and disposal and tertiary treatment would allow the plant to treat a further 35ML/d (resulting in a total capacity of 100ML/d). The capacity of the plant will therefore increase by 35%. The potential further upgrade 120 MLD in 2023 should be considered. Umgeni Water (UW) concurrently proposes the installation of an electricity cogeneration plant, which will generate power from the gas from the two anaerobic digesters currently on site. Currently, 25 000 kg/day of sludge is fed into the digesters, with methane-rich gas emitted during the digestion process. This methane-rich gas will be used to generate electricity (for the Darvill Wastewater Works) through a process referred to as co-generation, yielding 800-1000 kW of electricity per day. Details of the upgrades, which are relevant to the authorisation requirements, are as follows: Primary treatment (i) An additional 40m diameter primary settling tank with a surface area of 1 250m2 is likely. (ii) The construction of a new aerobic reactor (nominally 100 m x 60 m x 7 m deep) – 42 megalitres. (iii) Conversion of the existing aerobic reactor to an anaerobic / anoxic reactor. (iv) Construction of a blower house to provide air supply for a fine bubble diffused air aeration system. Secondary Treatment (i) Two additional 35 m diameter secondary settling tanks complete. Tertiary Treatment (i) Extension of the chlorine tank - the manner of extending the gas chlorination system has not been finalised. Sludge Handling and Disposal (i) The installation of an electricity co-generation process (Combined Heat and Power – CHP). The source of the biofuel for the process will be the methane-rich digester gas from the anaerobic digesters at Darvill Wastewater Works. With the installation of the CHP process, all of the methane-rich gas will be used to generate electricity (1 MW). (ii) Sludge generation will increase and the current forms of disposal may be insufficient and require revision. A study was conducted with the aim of finding an effective option for sludge disposal. After all possible options had been assessed , composting was chosen as a feasible option for sludge end use - the remaining sludge will be composted and disposed of for agricultural use. The above provides sufficient information on which to conduct the EIA and determine legislative requirements in terms of the Waste Management Licence and the EIA requirements. Regarding the water use licences, the current water use license issued for the plant restricts the discharge to a maximum of 32 million cubic meters of water containing waste per annum and this is based on an average dry weather flow of 75Ml/day. Once the plant is upgraded, it is anticipated that the sludge generated will exceed this limit and therefore a revision or new licence application will be required. Location: The Darvill Wastewater treatment Works is located off New England Road, within the Msunduzi Local Municipality. The WWTW is located on Portion 5 of ERF 1853.