Putting Waste to Work – Biogas to Energy
The City of Johannesburg’s water management company, Johannesburg Water (JW), has proven the business case for alternative energy solutions with the commissioning of a second municipal wastewater biogas-to-energy plant in Diepsloot. At the plant, more than 405Mℓ of sewage is treated per day and it has been in full operation since August 2012.
JW since commissioned a second biogas-to-energy project at its Driefontein waste water treatment works (WWTW) with operations expected to commence in October 2016.
Biogas-to-energy technology has the potential to reduce a WWTW’s reliance on conventional energy, but for some municipal plants, opportunities may be secondary to water treatment challenges, including sludge quality and plant management.
HARVESTING AVAILABLE TECHNOLOGY AND PROCESSES
These projects harvest biogas from the process of anaerobic wastewater sludge digestion.
“With rising electricity prices and load shedding, it became imperative to look for other solutions such as biogas capturing, upgrading and electricity/heat generation in combined heat and power (CHP) applications,” says Sofja Giljova, renewable energy advisor for the German development corporation’s (GIZ) South African-German Energy Programme (SAGEN).
“It makes sense for WWTWs greater than 25Mℓ per day to implement CHP solutions as this has the potential to reduce operational energy requirements, but there are also other considerations,” says Jason Gifford, biogas-to-energy business unit manager of WEC Projects, which was responsible for designing and developing both JW biogas-to-energy projects. The largest Northern WWTW plant is South Africa’s first municipal WWTW biogas-to-energy project.
“Biogas generated energy at source is a cheaper fuel for wastewater plants than grid purchased electricity. Yet, not all plants are economically
viable for CHP installations. The Green Drop status is a typical baseline evaluation of whether a plant may be feasible for a CHP installation,” says Gifford. Green Drop is a certification programme for wastewater quality management regulation.
NORTHERN WASTEWATER PLANT: OPERATIONAL PERSPECTIVES
The energy plant is fed biogas harvested from four digesters. It has a combined electrical power of three 376kW equivalent (kWe) CHP generator sets. The plant’s total installed capacity is 1128kWe.
“Over the four years of the CHP plant’s operation, its mechanical availability has been in excess of 98%. The Northern plant uses about 8MWe. Running at full mechanical capacity the plant could produce about 12.5% of the wastewater plant’s energy demands.
“The current infrastructure for digestion of the Northern plant sludge offers a current energy cost saving of between 15% and 20%,” says Ronell Viljoen of Johannesburg Water. “The first installation confirmed the potential of energy cost savings; biogas installations will form part of Johannesburg Water’s future upgrades and expansion to all large and macro-sized WWTWs.”
A critical challenge to overcome at the Northern plant is limited digester capacity relative to the volume of sludge available. JW is currently refurbishing two additional digesters with the expectation that six operational digesters will produce sufficient biogas to enable the CHP plant to run another generator set full time. The sludge in the digesters is heated with the recovered heat from the generator sets to ensure the digesters operate within a temperature band suited to the bacteria that feed on the sludge and produce methane-rich biogas.
JW acknowledges the financial factor. “Digestion capacity has to be in place before the biogas-to-energy plant can be up-scaled to reach full wastewater plant potential. But sludge treatment is expensive. Rolling out biogas-to-energy projects requires large capital investments,” says Viljoen.
ON-SITE ENERGY USE
“The biogas-to-energy generated at the Northern plant is used on-site. Very few plants will be in a position to export power to the grid or another third party. Although it may be a target for WWTWs to become energy independent through biogas-to-energy harvesting, the best case reality is around 60% of plant electricity requirements,” says Gifford.
There are two key factors restraining the biogas-to-energy generation potential for a WWTW. “Municipal plants in South Africa typically apply a biological nutrient removal (BNR) process to clean wastewater. This intrinsically reduces the amount of nutrient sludge available for digestion. Furthermore, this embedded part of the wastewater treatment process is energy intensive. Municipal plants are therefore only likely to supply, in part, their own energy requirements,” says Gifford.
From a strategic perspective, “it is advisable to apply biogas in CHP units for electricity and heat generation on-site, in the absence of a well-established gas grid in South Africa. Besides on-site electricity use, the heat produced can be used for digester heating, which makes the anaerobic process more efficient,” says Giljova.
CLIMATE CHANGE MITIGATION
Methane’s global warming potential is significantly more potent than carbon dioxide. “Most municipal WWTWs do not capture but emit the gas produced from standard wastewater treatment processes into the atmosphere. Biogas-to-energy projects can form part of national climate change mitigation strategies,” says Gifford. “These projects can furthermore eliminate approximately one tonne of carbon dioxide per megawatt of conventional grid electricity.”
INHERENT ADVANTAGES OF BIOGAS
“The key difference between biogas and other sources of renewable energy is its operational availability which is nearly 100%. Biogas energy generation may be more expensive per megawatt of generation capacity compared to large-scale wind and solar photovoltaic plants, but biogas energy generation can be used as baseload power or as peak power. Thus, providing the gas storage is adequate, energy is available on demand,” says Gifford.
The potential for biogas remains largely unexplored in South Africa. “From various projects and scales, only about 15MW of biogas generation capacity has been installed in South Africa to date, while the estimate is that there is around 3GW of electricity potential from biogas,” says Tiepelt.
CRITICAL OPERATIONAL CHALLENGES
Biogas-to-energy potential at WWTWs is plagued by challenges. “The reality is that municipalities are not developing these projects because there are critical issues that need to be solved first. It appears most municipal works receive diluted effluent due to system ingress,
from household to industrial level. Ensuring functional water treatment processes is another general and more critical challenge,” says Gifford.
Amid such critical everyday challenges, biogas-to-energy plants may be regarded as unessential. “If a municipal WWTW cannot meet its legally required effluent discharge requirements, the implementation of a CHP plant will not be a priority.” says Gifford. This is even more so the case if funding is not readily available.
SOFTENING MAINTENANCE IMPACT
Sub-contracting biogas-to-energy plant maintenance to external contractors can soften the operational impact for WWTW management. For both Northern and Driefontein biogas-to-energy plants, WEC Projects remains responsible for all operational and maintenance requirements.
Daily operation of the biogas-to-energy plants requires one operator, supported by an external maintenance and operational crew for servicing of mechanical and electrical components. Maintenance requires specific expertise, and SABIA foresees the continued need for international support. “The lack of sufficiently experienced local expertise for operation and maintenance remains a challenge,” says Tiepelt.
“The current day WWTW technology remains energy intensive. “But tapping into a natural by-product can reduce the operational energy costs,” says Viljoen.
Taking an example of the country’s largest WWTW on the Cape Flats in Cape Town: “The total savings through biogas-to-energy installation could amount to about R215 million in grid electricity consumption savings over the project lifetime,” says Giljova.
By Francini Van StadenThe full article appears in Earthworks Magazine, Issue 34: oct/nov 2016, pp 92-98