(示意圖/圖片來源:Vmenkov)

生質能結合水產養殖,不與農爭地、爭肥料,還能改善水質

當我們將農業廢棄物作為能源再利用時,必須考量廢棄物原本的使用途徑,是否可能因為處理方式的改變,而衍伸出其他問題?例如:稻桿原本作為土壤改良的堆肥,移除作為生質能後,必須解決土壤肥力減少的問題。但若以「水產養殖業」生產的生質能,就不會有改變土質或影響糧食生產的疑慮了!

在水產養殖系統中,養殖廢水含有高濃度的營養物質,當這些有機廢水與農田渠道合併流入環境中,濃度過高的營養鹽對水生生態系統具有危害,如藻華及優養化。若能事先去除營養物質,則可大幅降低環境生態損失,取出的營養物質也可轉換成生質能再利用。

藻類與水生植物生長速度快,自大氣中吸收二氧化碳並釋放氧氣,可減少溫室氣體。傳統上藻類必須從野外採集而來,但現在也可以藉由添加營養鹽人工養殖,再加工為生質酒精或其他生質燃料。目前水產養殖業數量保持穩定成長,若開發藻類養殖與水產養殖的整合系統,兩者皆會比單一系統生長得更好,動、植物可相互提供所需的營養素,並同時減少水產養殖廢水。


(魚菜共生示意圖/圖片來源:Ryan Griffis)

臺灣成功案例-龍鬚菜結合魚蝦養殖,同時產製永續性能源

臺灣因農業空間有限,較早投入開發藻類生質能系統,是成功開發水產養殖的國家之一。以人工種植而非捕獲的方式生產龍鬚菜(屬於紅藻, Garacilaria),將龍鬚菜與魚蝦一起養殖,再加工提煉為生質能,此生產方式對環境無負面影響,技術上可執行,且產製的能源具永續性。建立穩定的人工水生生態系統,在適當的時間點回收營養素,可以增加營養利用率及能源產量。

阿布達比也有生質能與水產養殖結合的成功案例,養殖廢水先經過鹽生植物(註)淨化,透過植物代謝攔截高濃度營養鹽,降低濃度後再排入環境,系統中的植物可收集轉化成生質燃料。

發展水生生質能源有助於環境永續,但實現整體經濟收益為正,仍待技術普及和應用推廣

以水生作物作為生質能源料有許多優點。其中最重要的是跳脫了傳統生質能的土地限制,不須與糧食作物爭地。除此之外,藻類的生長速度比任何陸生能源作物都快,因此具有發展潛力。而生物質吸收二氧化碳的特性,則可有效減少溫室氣體排放,降低我們對高碳排能源的依賴度。若將技術推廣至全球,可能降低全球的溫室氣體排放。

水產養殖能源系統是否成功的關鍵,在於經濟可行性。但由於技術尚待普及,有許多正面效益無法直接呈現在貨幣收益上,但若將所有環境效益列入考量,總體效益仍會高於投資成本,如:減少對化石燃料的依賴度、減少碳排放、提供穩定能源供給等,以及過濾處理養殖業廢水、防止優養化等優點。

臺灣有許多傳統的養殖方法,在水產養殖與周邊植物栽種等應用,找到兩者共存的平衡點,建立穩定獨立的運作系統,可做為未來水生生質能研究的參考。

註:鹽生植物(halophyte),指能在泥灘、河口等含鹽的環境中生長,並完成生活史的植物。

(責任翻譯:劉恩廷)
(責任編輯:羅時芳)

 


An Aquatic Approach: How Aquaculture Can Provide Environmentally Sound Clean Energy

Zoe Frye

While agriculture waste is a potential solution to finding a clean energy source, there are still problems which must be considered, primarily the disruption of the nutrient cycle and the removal of key nutrients from the soils relied on for food production. However, there is another type of agriculture which could provide the necessary biomass for fuel without taking over the limited space we have for food production: aquaculture. Nutrients are a product of aquaculture facilities, and when combined with farm runoff and organic wastes, aquatic ecosystems have acquired toxic levels. This abundance of nutrients can lead to severe problems such as algal blooms and eutrophication, which are harmful to both human and ecosystem health. By removing some of these nutrients in the form of algae and other aquatic plants, the chances of experiencing these problems is greatly diminished, and there is biomass available for energy production!

With available fossil fuel supplies depleting rapidly, it is time to turn to the original source of energy: the plant itself. Algae and other aquatic plants have rapid growth rates, remove carbon dioxide from the atmosphere, and release oxygen (Chan 1993). Farmed algae can be grown for many different reasons, including nutrition or aquaculture feeds, but it can also be a reliable source of energy by producing biofuels and ethanol (Food and Agriculture Organization). One of the most advantageous methods of aquaculture is developing an integrated system where plants and animals feed necessary nutrients to each other, which helps eliminate waste and grow better stocks of both (Chan 1993).

Like other aquatic resources, algae have been historically harvested from the wild. However, aquaculture is continuing to grow at a steady rate (Warwick 2016). Taiwan was one of the first countries to successfully develop an aquaculture farm, where Gracilaria is grown alongside fish and shrimp (van deer Meer 1983). Due to the limited agriculture space an island country can provide, Taiwan began looking towards algae as a source of biofuels (Kao 2008). Learning good techniques to farm, instead of capture, these energy sources will allow sustainability and availability to be maintained, and diminish negative effects on the environment (van deer Meer 1983). Learning about an artificial aquatic ecosystem that could provide biomass in healthy quantities is important, and learning when and how to properly remove and recycle nutrients can lead to increased food and energy, in both the aquatic and terrestrial terrain (Chan 1993).

In Abu Dhabi, a small scale integrated system was opened to figure out the technical aspects behind the science of sustainable aquaculture (Warwick 2016). The goal of the facility is to use the nutrient rich water in aquaculture systems and filter it through halophytes, which will create biomass to be converted into biofuel, while also cleaning the wastewater before it is released (Warwick 2016).

There are many incentives to developing biomass through farmed aquatic plants. One of the most important factors to consider is that utilizing aquatic ecosystems for energy crops does not take land which could otherwise be used for food crops (Kao 2008, Warwick 2016). In addition to utilizing a different space than food crops, algae grow faster than any land based energy crop (Kao 2008). This means that more energy can be produced solely on biofuel, diminishing the reliance on carbon. Algae also take in carbon dioxide, so in Taiwan the energy industry looks toward big industries to redirect their carbon emissions to feed the crops (Kao 2008). This could lead to a reduction of overall carbon emissions if globally initiated.

The key to a successful aquaculture energy system comes from developing technologies which can financially sustain it. However, there are many benefits which may outweigh the costs. Plants grown for biomass can help to alleviate the reliance society has on fossil fuels while also removing CO2 from the atmosphere, making it a far more stable energy source (Kao 2008). The plants grown can also help to filter waste water in order to prevent toxic levels of nutrients entering aquatic ecosystems (Warwick 2016). Many ancient practices that have been used in China and Taiwan have discovered ways to balance the plant material with the primary product (fish or seafood) in order to create an independently functioning system (Chan 1993, van der Meer 1983). By creating these sustainable artificial environments, it is possible to provide biomass fuel without further harming the environment through nutrient removal or ecosystem destruction.

References

  1. Chan, G. (1993). Aquaculture, Ecological Engineering: Lessons from China. Ambio, 22(7), 491-494.
  2. Food and Agriculture Organization of the United Nations. Aquatic Biofuels: Knowledge. http://www.fao.org/bioenergy/aquaticbiofuels/knowledge/en/
  3. Kao, C. (2008). Development of Biomass Energy in Taiwan: Current Status and Prospects. 4th Iasme/Wseas International Conference on Energy, Environment, Ecosystems and Sustainable Development (EEESD’08). 126-129.
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