International consulting engineering firm Royal HaskoningDHV has developed a tool that can measure an organisation’s carbon footprint to help reduce its emissions, says Royal HaskoningDHV principal engineer Siva Chetty.

The tool measures three different scopes of carbon emissions. Scope 1 emissions are those emissions an organisation is directly responsible for, such as fuel consumption. Scope 2 emissions are those measured according to the energy use of a company as supplied by a utility, such as State-owned power utility Eskom. Scope 3 emissions are those for which an organisation is indirectly responsible, such as those produced during the manufacturing process of chemicals that are bought by a wastewater treatment plant, explains Chetty.

The tool was developed with individual water treatment plants in mind, but it could be used to measure the carbon footprint of a multitude of treatment works in a municipality or of a water treatment plant and its network, he says.

Chetty notes: “Once a baseline is established, it is possible to create scenarios to reduce an organisation’s carbon footprint.

“The tool clarifies all forms of consumption, from paper and fuel to electricity and chemicals. With this tool, organisations can establish where the most emissions are generated and implement strategies to reduce them.”

Royal HaskoningDHV performed a computation for the eThekwini municipality in July, says Chetty, and a programme of intervention will be drawn up to lower its carbon footprint.

Royal HaskoningDHV market segment leader for water and wastewater treatment Bert Bakker says companies are apprehensive about implementing such programmes, as they are fixated on capital expenditure (capex).

Companies should take a life-cycle approach, where capex and operating expenditure are considered investments for long-term financial gains, he says. “Companies will spend money in the short term, but will receive the returns on capital investment quickly and, subsequently, record long-term financial gains,” he adds.

This life-cycle approach, Chetty notes, will be beneficial to companies, especially when South Africa’s carbon emission tax comes into effect in 2015. “The carbon footprint measurement tool will enable companies to save money by identifying processes or technologies with high carbon emissions for replacement with more efficient options with lower carbon footprints,” Bakker explains.

Energy Factory Philosophy
Bakker says the rise in electricity prices in South Africa has helped to create opportunities for alternative energy programmes. One such programme, which is spe- cific to wastewater treatment plants, is the energy factory philosophy.

“Europe has a head start in terms of investigating more energy efficient systems. Royal Haskoning-DHV is introducing the energy factory philosophy to the South African market using proven cases in the Netherlands as a reference,” he says.

This system regards wastewater treatment plants as more than just waste disposal facilities. It is a system where resources found in wastewater are harvested and used to create energy, explains Bakker.

“Wastewater treatment plants are regarded as being potentially energy neutral or energy producers with this system. All the energy needed in the wastewater treatment process can be derived from the wastewater. It is, therefore, possible to become energy independent, but a treatment plant can also produce energy for the surrounding stakeholders or the energy could be fed back to the national grid after signing a power purchase agreement,” he says.

There are many ways to produce energy, adds Bakker, noting that organic material from wastewater can be fed into anaerobic digesters to produce green gas, such as methane gas, and that many treatment plants use digesters in South Africa.

However, the green gas these digesters produce is only used to heat the digesters. The rest is being flared, in most wastewater treatment plants, instead of being used for other purposes, explains Bakker.

He says the efficiency of digesters can be increased, resulting in a much higher production of methane gas. This extra green gas is not just used to heat the digesters but can be used to power other equipment at the plant.

Bakker suggests that plants using digesters install cogeneration dual-fuel engines, which can burn methane gas to heat the digesters and generate electricity.

A treatment plant can also replace high-energy consuming equipment with more efficient options, thereby reducing energy use.

“Plants in South Africa have the potential to recover 50% of the energy used through the methane gas produced in the digestion process. Further, it is possible to reduce the remaining 50% of energy used by assessing high-energy consumers, like pumps and aeration blowers, and replacing them with more energy efficient solutions,” Bakker notes.

An energy-intensive process used by most South African wastewater treatment plants is surface aeration, which pushes air into water, feeding oxygen to the bacteria that break down organic waste. A mixer beats the surface of the water during the treatment process to force air into the water.

However, Bakker says surface aeration is an inefficient system and that
diffused aeration systems are more energy efficient because the air diffusers push tiny air bubbles to the surface and, owing to higher friction and a larger contact surface with the waste- water, more air is transferred into the water.

Further, he notes that 55% of the energy used at waste- water treatment plants is for the aeration process and, by replacing surface aeration systems with diffused aeration systems, plants can reduce their energy consumption by half during the aeration process.

“By doing this and using digesters and cogeneration dual-fuel engines, plants would no longer need to buy electricity. Further improving the digestion process will enable plants to produce electricity,” Bakker explains.

Chetty says, through the process of lysis, it is possible to increase the amount of organic matter or sludge that is treated in the digestion process.

“There are two types of sludge. Primary sludge has high potential to be used in energy production and is easily digested. Secondary sludge is produced after the aeration process and the organic waste in it takes longer to break down, making the sludge difficult to digest.

“Technology used in the Netherlands allows sludge to be hydrolysed at a high temperature and under high pressure, thereby making it more susceptible to the digestion process,” explains Chetty.

Through lysis or thermal- hydrolysis, secondary sludge is given the same energy potential as primary sludge, while increasing sludge production and reducing the waste of sludge.

Bakker highlights that the process of thermal-hydrolysis doubles the amount of sludge that can be treated in a digester tank. Municipalities can then consider treating other waste streams from nearby sources, such as factories, in their digesters, increasing the green gas produced.

“In the Netherlands, a company that transports such external waste streams to a treatment plant is retrofitting its trucks to run on liquefied methane, further emphasising the potential for energy saving and reducing carbon emissions,” Bakker notes.

Royal HaskoningDHV is working on six wastewater treatment plants in the Netherlands that are being converted into energy factories. The company has already been involved in the successful conversion of two treatment plants.