This case study was authored by the Australian Water Association’s Small Water and Wastewater Systems Specialist Network.
Traditionally in Australia we have adopted surface aerators for the aeration of earthen lagoon based wastewater treatment plants (WWTP). Around 90% of all Australian WWTP are based on earthen lagoons and aeration is the main energy consumer (around 60 to 70%) associated with the earthen lagoons treatment process.
Conventional surface aerators have oxygen transfer efficiencies of around 1.5 kg.O2 / kW.hr whereas fine bubble aeration has an oxygen transfer efficiency (depending on pond depth) greater than 2.6 kg.O2 / kW.hr.
If a fine bubble aeration system can be proven in a floating lagoon setting then significant overall energy savings are likely. Aeration upgrades to existing lagoons could also be completed within a week from equipment delivery to site with no process shutdown. This case study examines this further.
In May 2017, Goulburn Valley Water (GVW) agreed with Mapal Aeration on the terms for a pilot plant study of the Mapal Floating Fine Bubble Aerator (FFBA™) at their Tatura Wastewater Management Facility (WMF). This pilot study was to run for a period of 6 months. At the end of the study Mapal was required to remove the aeration and to source another use for the system.
To pilot Mapal FFBA at Tatura WMF and assess the effect on effluent water quality from the aerated lagoons, changes in power consumption and differences in Occupational Health and Safety (OH&S) requirements. The pilot was sponsored by GVW.
The WMF is located approximately 4 kilometres south of Tatura. The treatment process is lagoon based and consists of three high rate anaerobic lagoons (HRAL) operating in parallel series with three aerated lagoons and two facultative lagoons (see Figure 1). Two of the aerated lagoons (Lagoons 4 and 5) currently use 30kW low speed surface aerators, while the third (Lagoon 6) employs a 2kW pumped aeration system.
The facility receives approximately 2 GL per annum of domestic, commercial and industrial (trade waste) wastewater. Influent quality is reasonably consistent owing to the large proportion of trade waste received from industrial customers. Treated wastewater is reused via irrigation on GVW and third party properties, while biogas (methane) generated in the anaerobic process is sent to a third party electricity generator, or flared to reduce GVW’s annual greenhouse gas emissions.
Akuna Infrastructure Solutions (Akuna) completed modifications to the aeration installation at Tatura WMF. In 2017, two of the four aerators were shifted from Lagoon 4 to Lagoon 5. One of the four FFBA associated with the aeration system is shown in Figure 2.
After shifting two FFBA units, the existing surface aerator in Lagoon 5 was turned off. The image above shows an FFBA unit being lifted from Lagoon 4 via crane. Previously in Lagoon 5 GVW were using a conventional surface aerator.
On the right hand side of the image you can see a deposit of black mud from the bottom of Lagoon 4. Though difficult to confirm, it would appear that this material (black mud) is on the bottom of both Lagoon 4 and Lagoon 5. The black mud would appear to be Waste Activated Sludge (WAS), however, as there is no WAS removal in this aerobic pond (common for this type of treatment system) the WAS deposits on the floor of the lagoon. It can be resuspended if significant mixing energy is provided in the local area and can start to consume available oxygen.
Two FFBA units were craned into Lagoon 5. Initially it was not possible to float the FFBA units (i.e. they both ran aground). In contrast the FFBA units could float easily through Lagoon 4. As a result it would appear that less black mud is present in Lagoon 4 and may have been consumed by the improved aeration in Lagoon 4, though this is difficult to confirm. The volume of black mud should be the same between Lagoon 4 and 5 as they have both been receiving the same influent load.
To float the FFBA units in Lagoon 5, aeration was started with the FFBA units both having run aground. After about five minutes of operation the FFBA systems were able to be moved into position within Lagoon 5. It would appear that the mud re-suspended after aeration for a short period of time.
A custom automatic blue hook can be seen in the top of the image. This allowed the installation contractor to install the units within Lagoons 4 and 5 without entering the lagoons. Further information on these automatic hooks can be provided on request.
The following observations about the Mapal System in Lagoon 4 were made.
With no WAS removal, limited screening and no grit removal, a range of factors can effect the final treated effluent performance. At Tatura WMF one of the key treatment methods appears to be sedimentation given the pond dimensions of 90m x 280m x 1.5m and overall hydraulic retention time (HRT) of more than eight days.
When four FFBA units were located in Lagoon 4 it appears the significant increase in mixing energy may have resulted in a re-suspension of WAS, resulting in variable final treated effluent figures. This seems to be confirmed with the improved final treated effluent observed when only two FFBA units were located in Lagoon 4.
Based on the limited data gained from the Lagoon 5 trials, it can be seen from the trend above that there is no noticeable difference in filtered BOD removed before or after the FFBA units were installed. It should be noted that spikes in unfiltered effluent BOD were most likely attributed to microalgae and solids being resuspended by the MAPAL FFBA system.
The graphical data from Lagoon 5 in Figure 4 seems to produce similar effluent to the 30 kW surface aerator. Two FFBA units require only 15 kW of power for operation.
Improved DO residual/treatment
Lagoon 5 has returned to a ‘normal’ green colour, indicating the return of algal blooms of the type commonly seen in Lagoon 4.
At one stage in December prior to the Mapal system being installed, Lagoon 5 turned pink/purple due to organic overloading (Figure 6). This phenomenon has been observed at another WMF under similar stress, and was confirmed to have been due to organic overloading.
Furthermore, in early February 2018 GVW observed birdlife land on and swim in the lagoon, indicating that the water quality must be improving. Birdlife landing on these ponds has been rare.
There has been a noticeable lack of odour on the days that GVW has collected samples from Lagoon 5 once the Mapal system was placed in the lagoon. Without aeration, there is a great potential for odours to be generated. The improvement in aeration efficiency has seen odour from the pond reduced.
|Average Values (mg/L) 4th Jan – 14th Feb 2018|
HRAL 1 Discharge
HRAL 3 Discharge
The Tatura WMF pilot plant study was undertaken to assess the viability of the Mapal FFBA system in an Australian setting based on earthen lagoons and shallow pond depths (i.e. under two metres). GVW wanted to better understand efficiency gains with the system and the potential Operations and Maintenance (O&M) requirements of the FFBA system. The pilot trial was completed over a period of 6 months. The aeration system at Tatura WMF was then relocated from Tatura to Gisborne STP (Western Water) and has been in operation for more than 8 months with no reported issues.
It appears after 6 months of operation that the four FFBA units with a 30 kW blower produce a similar outcome (in terms of effluent water quality) compared to two of the 30 kW blowers (60 kW total) surface aeration capacity. On a power only basis this equates to the following saving:
30 kW (the saving) x 16 hr (hours of operation/ day) x 0.20 ($/kW.hr) x 365 (days per year)
Total saving per year = $35,040
If the system was adopted for the three aeration ponds the yearly power saving would be approximately $45,000 per year.
Given that the installation of the FFBA systems are similar in cost to a well made surface aerator, the potential power cost savings are significant for any lagoon based installation.
Occupational health and safety
Currently at the Tatura WMF site, staff go out onto the lagoons on a monthly basis to de-rag impellers and maintain gearboxes for each of the surface aerators. It is a required activity that raises a number of health and safety concerns and it would be GVW’s preference to avoid this work in accordance with a typical risk management hierarchy.
One of the other major advantages of the FFBA system is that no work is required on the water because all motorised parts are on the bank (i.e. in the form of an aeration blower). In fact, during the 6 month pilot trial period no contractor or GVW representative needed to go out onto the water to undertake maintenance activities on any of the FFBA units.
For GVW the potential to avoid lagoon access for maintenance on aeration equipment would take away one of the key business risks associated with wastewater treatment. It would appear to present a major non-financial benefit for GVW and could change the way it operates existing lagoon based treatment systems.
The potential of Mapal Floating Fine Bubble Aeration (FFBA) has been demonstrated at Tatura WMF. The FFBA would appear to provide significant power savings and OH&S benefits for GVW.
Power savings appear to be in the range of 30 to 40% but depend on the depth of the pond. Mapal’s experience shows that fine bubble aeration systems are even more efficient when aeration ponds are over 2.5 metres. This accords well with the theoretical figures shown in relevant engineering texts where fine bubble aeration efficiency is quoted at around 3.0 kgO2 / kW.hr.
The outcomes from this case study could be applied to other lagoons. Tatura WMF receives high strength wastewater from a number of industrial customers so it may even be possible to get better outcomes at other sites.
For GVW, the OH&S benefits are obvious, as previously operators were required to boat out to the surface aerators on a monthly basis to perform critical plant maintenance. During the 6 month trial of the FFBA system no operator or installer was required to enter the pond. All installation, commissioning and maintenance was done from the lagoon embankment. From an OH&S risk management perspective this was an excellent outcome.
For more information on the Australian Water Association’s Small Water and Wastewater Systems Specialist Network, click here.