Half a million tonnes of algal biomass a year.
I’m told that it was they estimate they are harvesting off Lake Tai in Wuxi.
Let’s do a quick back of the envelope calculation on that one, shall we? What area would we need to produce the same tonnage? Let’s be conservative, and say we’re talking wet weight, so divide by ten. For a benchmark productivity calculation, I’ll assume 20 tonnes per hectare per year. That works out as around around 2500 hectares. Or 3501 football pitches of algal ponds or bioreactors if you like your measurements soccer related.
Lake Tai is actually 225,000 hectares – meaning that the productivity is a lot less than our imaginary purpose-built facility, but those yields are still astounding. Probably a good point to ask why.
There are a number of causes of the algal blooms here, point source pollution being a primary one. This is a highly industrialised area, and factory wastewater combined with effluent from water treatment plants provides a bountiful feast of nutrients.In 2007 a severe bloom event occurred, which disrupted drinking water supplies for five days for 2 million people. This catalysed some of the measures to take action, including the harvesting facility we’re visiting today.
As I often explain to people, it really isn’t the fault of the algae. An analogy I like to use is that they are like students. Put some free beer in front of the quad, and they’ll show up in droves and hang around until it is depleted. Too much beer, and afterwards they might cause trouble. Too many nutrients, you ithe algae wreak havoc.
The most problematic of the photosynthetic invaders of Lake Tai are the blue-green algae, (cyanobacteria). According to the Redfield ratio, these are favoured when phosphate is more abundant than nitrate. Microcystins are toxins produced by the cyanobacterial blooms. They are hepatotoxic and are remarkably stable compounds which can persist over a ranges of temperatures and pH.
We drive to a pumping station where the blooms are channelled into ponds, and the biomass is aggregated using aluminium salts, and dewatered with what looks like a screw press filter. The algae is collected in bags, and given to farmers to use as fertiliser and it has been trialled as feed. They seem to get good results, according to the people on site. Given that the microcystins are so stable, I wonder if that is an issue when using it so near the food chain?
They tell me there is a collection station almost every kilometre, about 53 in the region that we are in. While wastewater standards are becoming stricter in China, monitoring is a major weakness it seems. You can’t get caught if you aren’t getting measured. And even getting caught might just be a minor inconvenience. Even though it is coming to the end of bloom season as the temperature drops, there is still a lot of algae to see. When light and temperature become limiting for the photosynthetic organisms, the reserves of nitrate will build up over winter ready to launch a new bloom in spring. The stations will continue to harvest the algae in an attempt to improve water quality, but ultimately their efforts need to be matched with action at the source. Until the excess nutrients stop entering the lake, the blooms will keep happening.
While China is now funding the measures to clean up with the algae collection facilities, those who have been speaking out about the problem haven’t been thanked by local government in the past. You can read about Wu Lihing here who lost his job, was harassed, and eventually landed in jail for his environmentalism.