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This Little Piggie Went Wee Wee Wee

Credit: dusanpetkovic1/Adobe

Credit: dusanpetkovic1/Adobe

By Jeremy Ayre & Navid Moheimani

Microalgae strains that can survive the extreme conditions in piggery effluent could not only clean up the wastewater but also reduce greenhouse emissions, provide a source of biofuel and even be fed back to the pigs.

Pork is the most highly consumed animal meat globally, with the pig population currently numbering around 900 million worldwide. Australia has roughly one pig for every ten citizens, so pig production here requires significant effort to keep environmental impacts in check while remaining economically viable.

Wastewater management is one of the important challenges. Piggeries are quite good at isolating their wastewater to prevent the contamination of nearby streams and groundwater, and this seems to be a proven part of the environmental strategy. The wastewater holding ponds utilised by piggeries also provide a treatment that involves anaerobic digestion of the waste to reduce the nutrient load and make the wastewater more manageable.

Pig producers are also moving toward covered ponds that can capture the methane produced and substantially reduce the associated odour. The methane can be a useful on-farm biofuel, and also allows for a reduction of greenhouse gas emissions as the carbon dioxide output from combusted methane is a comparatively less potent greenhouse gas.

Unfortunately there is still no economical and widely used option for treating the anaerobic digestate of the wastewater, apart from evaporative ponds that essentially allow the water to become lost into the atmosphere. This obviously doesn’t allow for very effective water recycling. This might offer some degree of water reuse if the water was clean enough to wash down the holding pens and help keep the piggery clean, but the high ammonia content of the anaerobic digestate prevents this. Obviously the more purified the water quality, the more diverse range of uses that are available.

While piggeries are doing the right thing by keeping the greenhouse gas emissions low and preventing water contamination, they are still missing out on the benefits of water reuse and have the additional problem of a very high nutrient load in the wastewater. The piggery anaerobic digestion effluent is therefore an interesting target for novel wastewater treatment technologies.

For some decades researchers have looked at treating the piggery wastewater with microalgae, but the research has been limited to tentative approaches using diluted raw wastewater or sometimes diluted anaerobic digestate. Some approaches have also looked at seawater enriched with the nutrient-heavy wastewater. These approaches relying on dilution are all based on the assumption that the high ammonia content along with the wastewater’s very dark colour (due to turbidity) make the medium highly toxic and almost impossible for algal growth.

So far these approaches haven’t brought about a change in the technology adopted by the pig industry. In most cases, particularly in Australia, dilution of the wastewater isn’t a viable option on a large scale.

Our research is attempting to utilise the wastewater as a beneficial resource rather than a burden. We have also isolated some microalgae species that can treat the undiluted anaerobic digestate of the wastewater.

The high nutrient load in the anaerobic digestate of piggery effluent makes it an interesting growth media for microalgae. Most microalgae cultivation methods depend on external sources of the critical nutrients such as nitrogen and phosphorous, but these nutrients levels are so high in piggery effluent that they are essentially toxic to most living things. Any microalgae capable of acclimatising to these conditions should therefore have a great advantage in being able to utilise extremely high nutrient levels as fuel for growth. If the issue of the turbidity can be overcome this also clears the way for growth in a very nutrient-rich growth media.

Our team is ramping up its research into the growth of highly acclimatised microalgae on undiluted anaerobic digestate. We have been able to isolate suitable microalgae from a wide variety of sources, such as an experimental piggery south of Perth that had microalgae growing in its evaporative wastewater ponds. By taking samples from this source and others we were able to obtain a handful of strains that can survive on the wastewater. In this case the only treatment prior to microalgae culture was a simple sand filtration process to remove suspended particles from the wastewater.

Following the successful cultivation at a base ammonia level of 700–1000 mg ammonia nitrogen per litre, we were able to demonstrate successful growth up to 1600 mg using synthetically added ammonia. This trial indicates that the microalgae can survive and grow at extremely high ammonia levels. No previous literature indicates growth at such high levels of ammonia.

Now that this groundwork has been laid, the mixed culture system can be optimised further. In a stepwise fashion we can start at the extreme top end of the conditions found in the anaerobic digestate and work towards understanding the limits to growth and economic barriers that might be associated with using the technology at the scale required in real-world piggeries.

Due to the extreme levels of nitrogen and phosphorous, we expect that other nutrients might be in limited supply and therefore need to be added to enhance growth. Control of the pH and the addition of carbon dioxide to the media also allow for some interesting tinkering with the system.

It’s certainly possible that the next generation piggery system will use anaerobic digestion to produce a significant biofuel in the form of methane-rich biogas. Indeed, piggeries are already putting these systems in place throughout Australia and other countries.

This also means that the CO2 output from biofuel combustion can serve as a pH control and additional carbon source to enrich the microalgae culture. Lowering the pH of the culture should lower the ammonia toxicity while at the same time giving the microalgae a boost of carbon.

We are also looking at different operating conditions, such as variations in pond depth and different bioreactor designs. Algae-harvesting might also play a role in maintaining a productive cell density.

Uses for the end-product algal biomass are also being investigated. A tantalising prospect is to consider using the algal biomass as a protein-rich feed supplement for pig production. Questions to address include any pathogenic properties of the algal biomass and whether the nutritional profile is favourable for pork production.

Another option being investigated with researchers at the University of Western Australia is the prospect of using the algal biomass to boost the methane output from anaerobic digestion. This would allow the algal biomass to feed additional carbon into the methane production system and improve biogas yields. Increasing interest in biofuels and decentralised energy production systems make this option economically compelling for piggeries.

However, these conditions are likely to produce tough algae, so these options may need a further process to physically break the tough cell wall to allow digestion, either in the gut of livestock or bacterial reactors.

With any luck the project will bring aspects of the technology closer to application for Australian pig production. We have excellent solar radiation in much of Australia, but the shortage of fresh water restricts the options available to dilute the microalgae culture in an anaerobic digestate production system. A preliminary economic assessment of all of the components still needs to take place to determine the technology’s overall viability.

Other anaerobic digestate treatments being investigated at Murdoch University are focusing on symbiotic associations between wetland plants and microalgae, comparing nutrient removal between open ponds and closed photobioreactors and comparing the effects on microalgae growth of paddle wheels and jets. Bioprospecting of microalgae is also being performed.

The research has international relevance due to the large scale of pig production globally, and has the potential to improve the economics of pig and other livestock production systems.

Jeremy Ayre is a PhD student supervised by Navid Moheimani at Murdoch University’s Algae R&D Centre.