Dry stacking can improve drying pan throughput but low solids concentration sludges will not stack. The project aim was to determine a cost-efficient way to modify conventional sludge handling in order to be able to stack low solids concentration wastewater treatment sludges. Full scale dry stacking trials were conducted to demonstrate appropriate design configurations and operational procedures.
The wastewater treatment industry often uses drying pans to stabilise and dry sludge; this method uses natural sunlight and wind effects to evaporate water from waste sludges, with very low energy input and low ongoing operational costs. However, this method does require a significant amount of land to be available over which to spread the sludge for evaporation. At present, the design at Melbourne Water requires one hectare of area to dry 400 dry tonnes of sludge from a solids content of around 3-5% by weight to about 70% solids.
In sludge drying, a crust will form on top of the wet sludge, which usually slows the drying of sludge underneath. An alternative operational technique to the current flooding practice is ‘dry stacking’, which has had large impact on slurry drying in the mineral processing industry, increasing the solids throughput by up to 300% compared to conventional tailings dams, substantially reducing the land area required. When operating in dry stacking mode, wet sludge is applied to the pan in very thin layers to the top of the ‘dry stack’. Thin layers of applied sludge are able to consolidate more than thick layers as the water in a thin layer does not need to migrate a significant distance to reach the surface and evaporate. This maximises consolidation during loading and minimises the amount of partially-dried sludge under the crust. The sludge layers can be applied in a way that a sloped sludge surface, or ‘angled stack’ is formed, able to shed rainwater, thereby increasing the effective net evaporation. It is estimated, based on local weather data, that dry stacking can improve the solids throughput of the sludge drying pans at WTP by 65 to 140%[.
This technology has not been used on a commercial scale for wastewater treatment sludges. The method was recently tested at Melbourne Water’s Western Treatment Plant (WTP) for the first time. During the trial, significant issues were encountered due to the low feed solids concentrations. These sludges required settling to form a stack, but the supernatant could not be decanted as the existing drying pan outlets rely on a certain water level inside the pan in order to drain properly. Instead, it stayed on the floor of the almost-flat pan, keeping the sludge saturated. Dry stacking of low concentration sludge requires the solids to settle and supernatant to be removed as soon as possible in order to expose the settled sludge to evaporation and allow the thin layer to dry somewhat before the next layer of sludge is applied. Effectively decanting thin layers of water through outlets in conventional sludge drying pans is a significant issue for application of the dry stacking operating method.
A key focussing question for the project was:
What cost-effective modifications are required to implement dry stacking of low concentration wastewater treatment sludges at WTP?
This work investigated various possibilities, including outlining a method for sourcing thicker sludge from the lagoons, options for increasing the solids concentration before reaching the pan, changes to drying pan outlet structures and in-pan biosolids embankments in order to direct sludge flow.
Stage 1 of the project included:
- Laboratory determination of the settling and decant rates for aerobic and anaerobic sludges at different solids contents and layer thicknesses to determine the range of flows that the outlets need to carry
- Cross-industry review to identify different outlet structure designs, with any necessary design alterations for wastewater sludges
- Proposed design alterations for wastewater sludge drying pans
- Design and scope of the structures/modifications for trial installation
The activities undertaken during Stage 2 of the project centred on full-scale trials at WTP and were:
- Install and trial modifications to conventional design to determine the most cost-effective way to facilitate the dry stacking method
- Determine the impact of rapid decant water removal on the dry stacking method through site testing and laboratory testing (including odour impacts)
Select a preferred outlet structure design and compile report on applicability of this design to conventional drying pans used in the wastewater treatment industry, including expected gains through increased net evaporation
The Anticipated Outcome
This project undertook full-scale trials at WTP that demonstrated a method to form dry stacks with low concentration sludges through the use of in-pan biosolids embankments. The operating procedures were documented and are currently being implemented at WTP. Despite its success, stacking low concentration sludges remains difficult and the long-term solution at WTP needs to have higher and less variable pan feed concentrations. The options of sourcing thicker sludge, concentrating the sludge before it reached the pan and using ditches in the pan were also considered. The full details of the project are given in project reports, delivered for each project stage, and a photographic ‘virtual tour’ of the trial site, both of which are available through the Smart Water Fund. This work also complements an ongoing ARC Linkage Project investigating the fundamentals of sludge stack formation and operational optimisation.
The cost savings associated with the successful implementation of dry stacking are a game-changer for solids handling in the wastewater treatment industry. At WTP, it is estimated that dry stacking will provide immediate capital cost savings of $30 M and ongoing drying pan operational cost savings of 30 % by reducing the size of sludge drying upgrades required in Stage 1 of Water Plan 3. There are further savings through deferred costs in Stage 2 in Water Plan 4 and beyond. Current investigations are underway to implement dry stacking at Eastern Treatment Plant, which also has significant capacity issues, reaping further benefits for Melbourne Water. There has also been significant interest from other metropolitan and regional water utilities throughout Victoria.
In general, any wastewater treatment plant currently using drying pans could upgrade their pans (with just minor changes) to utilise dry stacking and increase throughput, thus deferring upgrades and providing operational flexibility. In addition, plants that would otherwise require more costly, advanced dewatering and drying could potentially consider the more efficient drying pans as a low-energy, cost effective alternative, if some land was available. Further afield, dry stacking could be implemented in other industries that use solar drying for biosludges such as agriculture and aquaculture.