Anaerobic Digester Wastewater Management During Biogas Production
Targeted and Comprehensive Solutions
April 28th 2020
- Digester wastewaters are by-products of biogas production in anaerobic digesters. They require treatment prior to disposal. To meet regulation compliance, treatment options range from minor interventions such as selective contaminant removal, to major interventions such as minimum and zero liquid discharge (MLD/ZLD).
- Ammonia in digester wastewaters can be selectively removed by stripping, biological treatment, and chemical methods. Selection of options requires careful consideration of local conditions, energy sources, economic usability of the end-products, and most importantly the specific wastewater chemistry, such as ammonia concentration and co-contaminants (e.g. bicarbonate, phosphate, organics, and metal ions).
- An experienced wastewater treatment team can help you to review and economically assess the treatment options for digester wastewater, from selective contaminant removal to closed-loop ZLD, and everything in-between.
Anaerobic Digesters & Wastewater
Biogas is primarily composed of methane (CH4) and carbon dioxide (CO2). It is produced in an anaerobic digester (AD) using a biological process, degrading organic matter in the absence of oxidant (e.g. oxygen). Figure 1 below is a simplified PFD of biogas production with conventional digester wastewater management.
While some of the organic matter will be broken down into biogas, digestate slurry remains. It is a mixture of solids (leftover organic matter, biomass) and wastewater, in which inorganic salts are dissolved. Solids are separated and sent for disposal/used as fertilizer.
However, without further treatment the digester wastewater cannot usually be re-used directly or discharged into a public sewer. Typical issues include total suspended solids (TSS), total dissolved solids (TDS), ammonia, biological oxygen demand (BOD), and chemical oxygen demand (COD). These characteristics of the digester wastewater are impacted by the composition of the organic matter feed, for example: municipal organic waste, livestock waste, poultry waste, manure etc.
Digestate Wastewater Management
As in other wastewater treatment processes, such as for landfill leachate, digester wastewater treatment involves the application of various technologies and can range from highly targeted selective contaminant removal, up to a comprehensive ZLD process. Figure 2 shows an overview of a comprehensive treatment solution for digester wastewater, including options for zero/minimal liquid discharge (ZLD/MLD).
The most common digester wastewater constituents-of-concern are fine suspended solids, ammonia, and salt ions. However, depending on wastewater characteristics, and project requirements, not all treatment steps may be needed. For example, a low-strength wastewater, with only ammonia concentrations beyond discharge limit compliance, will only require ammonia removal technology. To decide on an appropriate and cost-effective treatment process, it is critical to know the wastewater chemistry in detail and understand the treatment requirements. Saltworks can review water chemistry and specific project requirements so that a digester wastewater treatment process is delivered that balances simplicity, cost-effectiveness, and reliability.
Removing Suspended Solids
Suspended solids in digester wastewater pose scaling/plugging problems in any downstream equipment, especially for ammonia removal and total dissolved solids (TDS) treatments. Additionally, suspended solids make up a significant portion of BOD and COD. They can be removed from the digestate wastewater through traditional physical/chemical separation processes, namely coagulation-flocculation using ferric chloride and polymers, followed by clarification/sedimentation and media filtration of the clarifier overflow.
Alternatively, a more advanced membrane technology, XtremeUF, can be used to remove suspended solids in the digester wastewater without the use of coagulation-flocculation. XtremeUF, shown in figure 3, is a ceramic-based ultrafiltration module that can concentrate suspended particulate into a dense slurry. It is highly automated, compact, and modular, offering an alternative to traditional physical/chemical separation processes for projects requiring low footprint, low maintenance, and process simplicity. A more detailed explanation on ceramic ultrafiltration can be found here.
The selection of an appropriate ammonia removal method depends on the concentration, the presence of other contaminants (e.g. bicarbonate, phosphate, organics, and metal ions), local conditions, energy sources, and the economic usability of the end-product (i.e. can it be re-used as a fertilizer). An overview of these ammonia removal options is provided in Table 2, below.
|Technology||Process Description||Considerations & Applicability|
- Bicarbonate is removed as CO2
- Base is added to increase pH
- Wastewater and air flow countercurrent in a stripping tower, ammonia is stripped from the wastewater into the air stream
- The air stream then enters a scrubbing tower and flows countercurrent to a sulfuric acid stream
- More economical for high ammonia concentrations (>2,000 mg/L N)
- Ammonia can be recovered as ammonium sulfate fertilizer
- Small footprint, widely practiced
- Acid/base consumption can be high
- Potential for calcium carbonate and/or calcium sulfate scaling in the stripping column
- High temperature steam flows countercurrent to the wastewater stream, ammonia and bicarbonate are stripped out of wastewater as ammonia gas and CO2
- Ammonia gas, carbon dioxide, and water vapor are condensed out as ammonium bicarbonate solution and/or ammonia solution
- More economical for high ammonia concentrations (>20,000 mg/L N)
- Can recover ammonia as ammonium bicarbonate solution or ammonia solution without chemicals
- Wastewater TDS also reduced, easing the load/cost of any downstream TDS removal process
- Steam stripping & condensation can be further compacted with a mechanical vapor recompression system
- Market share of ammonium bicarbonate or ammonia water fertilizer is smaller than ammonium sulfate fertilizer
- High energy consumption for steam and ammonia vapor condensation
- Nitrification: Ammonia is biologically oxidized into nitrate by bacteria in aerobic conditions
- Denitrification: Nitrate is biologically reduced into nitrogen gas under anoxic conditions
- Typically used for lower ammonia concentration wastewaters (<500 mg/L N)
- Ammonia is converted to nitrogen gas, no additional ammonia-based salt to manage
- Organics, phosphorous, and other co-contaminants are also removed
- Requires large footprint
- Large volume of sludge generation
|Anammox||- Ammonia is directly converted into nitrogen gas under anaerobic conditions||
- Best suited for ammonia concentration 500–2,000 mg/L N
- Ammonia is converted to nitrogen gas, no additional ammonia-based salt to manage
- More energy efficient and less sludge production than biological nitrification-denitrification process
- Slow startup and extremely temperature sensitive
|Breakpoint Chlorination||- Ammonia is chemically converted into nitrogen gas with the use of bleach (hypochlorite)||
- Best suited for low ammonia & organics
- No temperature sensitivity
- Simple installation, minimal start up time, and fast ammonia removal rate
- Bleach consumption is expensive; organics will consume bleach, increasing chemical costs
|Struvite Precipitation||- Ammonium is chemically precipitated out as struvite (magnesium ammonium phosphate)||
- Best suited for high concentrations of phosphate
- Struvite fertilizer market share is smaller than other ammonia-based fertilizers
Total Dissolved Solids (TDS) Removal Through Reverse Osmosis and Evaporation
The most common TDS removal technologies are reverse osmosis (RO) and evaporators. Due to wide availability, low cost, and energy efficiency, RO is typically recommended upstream of evaporators. You can read more about how reverse osmosis can decrease treatment costs here.
Digester wastewater enters a seawater reverse osmosis (SWRO) system for TDS and further organics removal. The SWRO operates at pressures up to 1,200 psi, producing a brine stream with around 70,000 mg/L TDS and a high quality permeate water stream. In a typical biogas plant, the SWRO operates at 80% permeate recovery. The permeate can be re-used within the biogas plant or be safely discharged. The SWRO-brine can be disposed of in the compositing process of digestate solids waste, or in a disposal well, depending on which option the economics favor.
If MLD/ZLD is required, the SWRO brine can be further treated in an ultra-high pressure reverse osmosis system, such as our XtremeRO system, shown in figure 5. XtremeRO uses spiral wound RO membranes that allow operation up to 1,800 psi and can further concentrate the SWRO-brine, up to a TDS of about 140,000 mg/L in some conditions. Compared to existing thermal evaporation technology, XtremeRO is about 3× more energy efficient for decreasing brine volume, providing a significant decrease in the energy cost of a downstream evaporation system. Saltworks’ intelligent automation for SWRO and XtremeRO make them especially well-suited for the treatment of digester wastewater. Flux monitoring and automated self-cleaning ensure that the membranes remain free of organic fouling.
To achieve ZLD, XtremeRO brine is then treated by a thermal evaporation system. Many options exist for evaporator systems, such as Saltworks’ SaltMaker AirBreather and MultiEffect products, shown in figure 6. The distillate from the evaporator crystallizer will be high quality and can be discharged or re-used within the biogas plant. All contaminants will be reduced to minimal liquid volume or solids for final disposal or land application.