Wastewater treatment plant design evolving as demand for energy efficiency grows
02 June 2015
Author: Michael O’Sullivan, chartered engineer, is the engineering lead with EPS Group’s Large Contracts division
The rapid growth of demand for energy efficiency has been the most significant change in wastewater treatment plant design since the introduction of design-build-operate contracts some two decades ago. This need to improve efficiencies has led to many changes in traditional thinking and a consequence has been the emergence of many new technologies from entirely new treatment processes to simple equipment adaption.
Two examples of these changes are described below as a means of illustrating the present changes in the wastewater sector. These are an introduction to the new ground-breaking Nereda® biological treatment process and the trial results of the BioCrack®, a new waste-to-energy device used to improve biogas yields from anaerobic digestion at wastewater treatment plants.
Nereda revolutionises wastewater treatment in Ireland
EPS are presently in the construction phase of wastewater treatment plants at Clonakilty (20,500PE – Population Equivalent) and Carrigtwohill (30,000PE) with both plants due to take flows this year. These are the first plants in Ireland to be built utilising the innovative Nereda biological treatment system. This technology is set to displace the century-old activated sludge process that forms the basis for most modern wastewater treatment plants.
Nereda is an innovative, sustainable and cost-effective biological wastewater treatment technology that purifies water using the unique features of aerobic granular biomass. The success of Nereda is its high water treatment capability in combination with significantly lower investment and operational costs, a very small physical footprint (up to a factor four smaller) and high energy savings (25 to 35 per cent).
Nereda treats wastewater with aerobic granular biomass: purifying bacteria that create compact granules with superb settling properties. The technology was discovered by the Delft University of Technology in the Netherlands, and developed in a unique public private-partnership between the university, the Dutch Foundation for Applied Water Research (STOWA), the Dutch Water Boards and Royal HaskoningDHV. After 20 years of research and development this innovative biological solution is now proving itself to be a leap forward in wastewater treatment technology.
What makes the Nereda process so special is the aerobic granules (biomass). In the usual treatment process, flocs of sludge are suspended in the water. It takes time for these flocs to settle in the treated wastewater A special secondary settling tank or clarifier is needed. Nereda granules settle much faster, and can do so in the same tank in which the treatment process takes place.
Owing to their composition, the granules have an aerobic outer shell which is rich in oxygen and an anoxic/anaerobic core that is low in oxygen. This means that we can find different treatment conditions in a single granule. In a normal treatment plant, the high-oxygen and low-oxygen processes take place in different parts of the overall process.
Nereda is being used internationally for the sustainable and cost-effective treatment of industrial and domestic wastewater. With tank sizes already similar to the world’s largest SBR-tanks, the technology is proven and suitable for even the largest applications. The performance of Nereda plants is outstanding and exceeds those of activated sludge. For example, the effluent of the Epe plant (pictured below), designed for flows up to 1,500m3/h, shows that Nereda even exceeds expectations from the baseline targets set.
The energy consumption of the WWTP is significantly less than any type of similar-sized conventional treatment plant in the Netherlands. During the Epe WWTP process proving period in 2012, the energy consumption per removed pollution equivalents (of 150g total oxygen demand) was recorded as 22.2kWh/(PE.annum) @ actual load. This is a 40 per cent energy consumption reduction compared with the benchmark for similar Dutch treatment plants with post-treatment (Union of Dutch Water Boards 2009) of 37.5kWh/(PE.annum) @ actual load. Furthermore, the effluent quality meets the highest standards, i.e. total nitrogen and phosphorous concentrations lower than five and 0.3mg/l.
The robustness and stability of the treatment process under strong varying influent load conditions and extreme influent pH fluctuations is remarkable. Furthermore, even during wintry conditions, extensive nitrogen removal could be established at very high biological sludge loads.
The introduction of this technology into the Irish market marks a significant milestone and confirms Ireland’s position as one of the leading embracers of modern wastewater technologies. Nereda will help make the Irish wastewater sector have a more sustainable future.
Enhancing anaerobic digestion
The EPS Group builds and operates anaerobic digestion systems in Ireland at multiple municipal wastewater sites. Anaerobic Digestion is a process for the stabilisation of solids and biosolids. This process involves the decomposition of volatile organic matter (principally sulphate) in the absence of molecular oxygen.
Because of the emphasis on energy conservation and recovery and the desirability of obtaining beneficial use of wastewater biosolids, anaerobic digestion continues to be the dominant process for stabilising sludge. Furthermore, anaerobic digestion of wastewater sludge can produce sufficient digester gas to meet a large portion of the energy needs for plant operation.
The Drogheda wastewater treatment plant (WWTP) and Dundalk WWTP were two large-scale facilities that came under special focus by the EPS Energy & Environment Committee in 2009. As part of this process, that included multiple energy and water consumption control measures, the abilities of the sustainable energy sources on site were investigated in order to minimise the plant’s reliance on external power sources.
It was clear that optimising and efficiently utilising the existing biogas systems was critical to the plant. The tracking of these usage figures led EPS to further investigate optimisation techniques for increasing biogas production in existing anaerobic digesters. To that end, EPS combined with an existing innovation partner, Vogelsang. The selected technology was called the BioCrack system and had been used in agricultural digestion applications in Germany since its release in March 2010.
The BioCrack unit operates by incorporating an electro-kinetic disintegration stage into the typical treatment process of a wastewater treatment plant. Electro-kinetic disintegration is achieved by first passing the digested sludge through a highly efficient macerator to mechanically break up fine solid particles and biological material in suspension. Once the solid components within the sludge have been reduced and broken down, it is then transported by pulsation-free and shear-free pumping past a series of high voltage AC probes that discharge a powerful electro-kinetic force through the sludge.
At this point a high voltage field is generated in the area surrounding each probe that causes the cellular membranes to rupture, releasing cell content into the sludge. As a direct result of this action the composition of the sludge is altered, leading to a greater availability of nutrients in suspensions for the population of bacteria in the digester to utilise during fermentation. This creates a highly active substrate for fermentation. The theory is that the sludge will decompose at an increased rate, the digester volume will be utilised more efficiently and the biogas yield will increase.
Throughout the duration of the trial at Drogheda WWTP during May and June 2011, operations staff continued to carefully monitor the level and quality of biogas production, pH, acetic acid and temperature levels, BOD and suspended COD and determined the percentage dry solids of sludge and centrifuged cake.
The volume and composition of biogas produced during the trial was compared against the volume of biogas produced previously during periods when the digester was functioning under similar environmental and operational conditions and the results showed the biogas volume increased dramatically and biogas composition had shown a decrease in carbon dioxide levels and an increase in methane levels.
Onsite observations on the performance of the plant highlighted that the digested sludge material had become less viscous as the trial period progressed. An increase in the heat transfer rate was also noted at the CHP units, suggesting that it was possible to heat the digester contents more easily as a result of the reduction of the viscosity of the digested sludge.
A decrease in percentage dry matter was also witnessed as the digested material was passed through the BioCrack unit. And, since the introduction of the new system, there was also a significant reduction in the volume of waste sludges produced from the digestion process, which in combination with the lower dry solids levels suggested a heightened volatile solids destruction rate within the digester.
Following the successful trial period, EPS installed this technology permanently at Drogheda WWTP as well as at Dundalk and Tullamore WWTPs which also contain anaerobic digesters. As expected, the volume of biogas produced was not as extensive as Drogheda due to the higher sludge residence times (Drogheda had only 14 days’ retention as opposed to 20 plus days at Dundalk, for example) allowing more time for natural digestion to progress.
The results still show that there is a significant cost saving to be gained from the incorporation of a BioCrack unit to the treatment process. Since the disintegration process uses just a powerful electrical field but little electricity, 35 W per electrode unit, the additional energy consumption of the plant is negligible. It is estimated that using SEAI conversion rates, the BioCrack systems presently contribute to a carbon reduction in excess of almost 500,000 tonnes of CO2 per annum to EPS on the operation of these digesters alone.
It is noted that EPS received partial funding from the SEAI under its Better Energy Workplaces Programme 2011 for this project which went on to win the 2012 Green Innovation Award. This system is an example of a customer-desire-driven, innovative design, reflective of the importance of creative thinking when energy saving measures are investigated in existing technologies.
Michael O’Sullivan, chartered engineer, graduated from Cork Institute of Technology (CIT) and is presently the engineering lead with EPS Group’s large contracts division. He has been operating in the water and wastewater sector in Ireland and abroad for more than 14 years. He is also chair of the EPS Energy & Environment Committee since its formation in 2009.
Appreciation is extended to Royal HaskoningDHV and Vogelsang for their support in the delivery of the above projects