Optimising carbon harvest from WWTP influents
Exploring the potential of counter current AB process (CCAB) applications in Denmark
Modern wastewater treatment requires significant amounts of energy for pumping and aeration. Fortunately, utilities can claim major savings by improving treatment efficiency and increasing on-site energy production through anaerobic digestion of the sewage sludge.
The CCAB solution increases primary COD harvest, with potential benefits in terms of biogas production and aeration requirements for COD conversion in the biological stage of the WWTP. The solution involves:
From 30% to >60%
Large savings and short Return of Investment times based on model simulations
CCAB operation established within 6 months
Utilities becoming energy neutral
The Danish Water association performs a yearly benchmark of Danish wastewater treatment plants (WWTPs), which has showed that over the last few years wastewater treatment has become energy neutral at an increasing number of utilities. This tendency is in line with expectations from local and national political goals and is expected to continue.
Continuous development of new technologies to further reduce energy consumption and increase production is needed to achieve this goal. The main source of input for locally produced biogas is COD in the primary sludge.
Existing technologies for increased COD harvest are costly and require high maintenance
Currently, two commercially available technologies for increased COD harvest from municipal wastewater dominate the market. These are fine-mesh sieving (filtration) and chemically enhanced primary clarification. Both solutions offer increased COD harvest compared to traditional primary clarification, but unfortunately also include significant downsides.
Filtration requires advanced machinery with timely and costly maintenance, while chemically enhanced clarification produces a significant amount of inert chemical sludge that does not contribute to biogas production and generates additional load on the sludge handling systems.
Expanding highly loaded plants is expensive and not always physically feasible
Wastewater treatment plant loads tend in time to diverge from the designed capacity of the WWTP. Expanding the capacity by building additional infrastructure, such as process tanks and clarifiers, is quite expensive and urban areas are often limited in vacant space for the expansion.
The CCAB process
To improve the wastewater treatment process, the CCAB technology features pumping of activated sludge from the secondary clarifiers to a contact tank located prior to the primary clarifier. The activated sludge acts as biosorption medium for COD in the influent untreated wastewater. In the primary clarifier, sludge particles settle together with a large fraction of the biologically sorbed COD.
Figure 1. Process flow diagram of the CCAB process. © DHI
Lab-scale tests to determine the feasibility of the technology
Laboratory tests were carried out to determine the potential of CCAB operation before full-scale implementation and during full-scale operation. Good correlation was shown between lab and full-scale results. This allowed DHI to use lab-scale tests as an indicator of COD harvest potential of the technology.
Full-scale implementation and monitoring
The CCAB process was implemented and operated in full scale at Esbjerg East WWTP in Denmark. Advanced online process control was installed to ensure stable operation, building upon more than 30 years of experience with wastewater process control at DHI. Sampling campaigns were performed to determine the actual COD harvest and used as input to model the CCAB process.
Simulations used to determine business case
The modelling software WEST was used to run simulations of the baseline (current) operation of four wastewater treatment plants in Denmark and compare it to three alternative technologies with primary COD capture. The simulated scenarios included filtration, chemically enhanced clarification and CCAB systems. The endpoints for evaluation and comparison of the different scenarios were: (i) COD load provided for anaerobic digestion; (ii) power consumption for aeration; (iii) external carbon dosing to meet denitrification needs.
Improved COD harvest
Lab-scale tests and full-scale operation showed comparably high COD harvest in the primary stage by utilising the CCAB process. Namely, 62% of incoming COD was removed in the primary clarifier as a result of biosorption and separation. This number compares favourably with other commercially available technologies.
Potential for increased efficiency
Based on WEST simulation results, the CCAB process showed a positive impact on the energy production due to a doubling of the COD harvest in the primary stage, as compared to baseline operation. Furthermore, removing COD in the primary stage proportionally reduces aeration needs in the biological stage. A comparison of the test period with a similar period during baseline operation showed approximately 40% increased energy efficiency when looking at COD removal.
Favourable business case and increased capacity
The business cases evaluated as part of the project, favour CCAB operation in three out of four plants. CCAB operation is economically feasible at plants with existing primary clarifiers (ROI < 2 years) but also in cases where primary clarifiers would have to be built (ROI < 6 years). In all three cases, the yearly savings were higher for CCAB compared to any other scenario. Furthermore, improved COD harvest reduces the process tank volume needed to remove the residual COD through aerobic conversion, giving plant operators additional freedom in managing their plant.
Figure 2. Difference in operational cost, as compared to baseline operation, resulting from the implementation of COD harvest technologies. Negative values denote decrease in operational costs, thus savings.
Eight Danish utility companies and the Danish Water Association joined forces and formed the Wastewater Partnership under the Market Development Fund to support research in the area of resource and energy extraction from wastewater. Three technologies were developed, among them the CCAB process.
WEST - DHI’s state-of-the-art software for modelling of wastewater treatment processes and evaluation of alternative operational scenarios.
