Valorization of Bokashi leachate as feed additive in tilapia farming

Highlights

• First study to exploit the potential usage of Bokashi leachate (BL) in fish farming.

• BL dietary inclusion enhanced growth of tilapia fed soybean meal (SBM) based diet.

• Growth of fish fed BL added diet was comparable to that fed fish meal based diet.

• BL inclusion reduced the crude fiber content of SBM diets.

• This study has achieved the goal of “Turning Waste to Gold”.

Abstract

The Bokashi leachate (BL) is a by-product from the anaerobic digestion of food waste, following the Bokashi composting method. Bokashi leachate is acidic and it contains effective microorganisms hence it has potential to be a functional feed additive to the plant proteins based diets for fish farming. This study evaluated the growth performance and feed utilization of the red tilapia (Oreochromis sp.) fingerlings fed with the BL supplemented soybean meal (SBM) based diets. After an 8-week feeding trial, fish fed with the 5% BL supplemented SBM diet attained the highest weight gain. This result was significantly higher (p < 0.05) than those fed with the 0% BL supplemented SBM diet, and comparable (p > 0.05) to those fed with the control full fish meal (FM) diet. Generally, dietary inclusion of BL enhanced the fish feed intake on the SBM diet but it did not show clear sign of improvement in their feed utilization. In addition, no significant difference was found across the hepatosomatic index and viscerosomatic index from all dietary treatments. These outcomes concluded that dietary inclusion of BL can enhance the feed intake and growth performance of the red tilapia fingerlings fed with the SBM based diet without compromising their health, and the optimum BL inclusion level was 5%. Nevertheless, further study on the properties and substances content of the BL produced from different types and ratios of food waste is strongly recommended. In this study, BL was also discovered to be capable of reducing the crude fiber content in the SBM diets. Such observation deserves a further exploitation on the application of BL to manipulate the crude fiber content in the plant proteins based diets in fish farming.

Introduction

Management of food waste has always been a challenge for most of the countries in the world (FAO, 2011; Garcia-Garcia et al., 2017). Landfill is the most common method to manage food waste but such method is not environmental-friendly and poses high risk to human health (Thi et al., 2015); the landfilling food waste is biodegradable hence it can result disease vectors (Louis, 2004) and increase greenhouse gas emissions (Adhikari et al., 2009). Anaerobic digestion is a environmental-friendly alternative that has been widely applied in European Union and Asian countries to manage food waste (Abbasi et al., 2012). Through this method, biogas and the nutrient-rich fertilizer can be generated (Christian, 2007).

The “Bokashi composting” is an anaerobic digestion method widely practiced in Japan and Southeast Asian countries to produce fertilizer or organic matters from food waste through fermentation by effective microorganisms, comprising mainly the selected species of lactic acid bacteria, purple non-sulphur bacteria, and yeasts (Footer, 2014). After 2-week of composting, the fermented food waste can be used as the Bokashi fertilizer by incorporating them into soil through the trench method (Footer, 2014). The Bokashi leachate is produced as a by-product from the fermentation process. It is a yellowish or orangish acidic liquid (pH 4.5–6) with the smell of sweet or sour vinegar and containing effective microorganisms (Footer, 2014). Studies have reported on the positive effects of Bokashi fertilizer and its leachate to improve soil fertility for higher plantation yield (Xu et al., 2001; Mayer et al., 2010; Zaman et al., 2010; Murillo-Amador et al., 2015; Álvarez-Solís et al., 2016; Quiroz and Céspedes, 2019). However, there is still no scientific exploitation on the potential use of the Bokashi leachate in fish farming or aquaculture application.

In aquaculture industry, feed cost is the largest expenditure in the total cost of fish production (Rana et al., 2009). The high feed cost is due to the expensive fish meal (FM) which is traditionally used as the major protein source in fish diets (Tacon and Metian, 2008). Indeed, the FM price was about USD 1500 to USD 1600 per tonnes in 2019 (FAO, 2020). To reduce the feed cost, researchers are searching for alternative protein sources to replace FM. Soybean meal (SBM) has being widely accepted as an alternative due to its high protein content, favorable amino acid profile (McGoogan and Gatlin, 1997; Kikuchi, 1999), lower price than FM, and high availability (Hardy 2006). However, the dietary inclusion of SBM can cause several problems, such as low feed intake and poor nutrient digestibility which can lead to poor growth performance and feed utilization in the farmed fishes (Krogdahl et al., 2003; Gatlin III et al., 2007; Hardy, 2010; Shapawi et al., 2013; Lim et al., 2014). The use of supplementing feed additives, especially organic acids, can ease those problems (Ng and Koh, 2017).

Red tilapia, (phenotypically Oreochromis niloticus) is one of the most commonly cultured freshwater fishes, especially in Malaysia. The aquaculture production of tilapia covers about 31% of the total freshwater aquaculture production (Fisheries Department of Malaysia, 2018). This production worths approximately RM 324 million, about 46% of the wholesale value of the total freshwater aquaculture production (Fisheries Department of Malaysia, 2018). Nevertheless, feed cost still comprises about 60% of the total production cost of tilapia in Malaysia (Ng et al., 2013), although the commercially available practical diets for tilapia in market already contain about 30% of SBM protein. Dietary supplementation of organic acids or organic salts generally can improve the growth performance and feed utilization of tilapia (Ng et al., 2009; Abu Elala and Ragaa, 2015; Koh et al., 2016; Soltan et al., 2017; Huan et al., 2018), however, such practice may be costly especially to the small scale farmers (Ng and Koh, 2017).

Considering that Bokashi leachate contains effective microorganisms and many micro-nutrients from fermentation of food waste, and it could have pH level similar to organic acids (Footer, 2014), we hypothesize that Bokashi leachate can potentially be a suitable functional feed additive to the practical diets for red tilapia. There has not been any literature reported on the potential usage of the Bokashi leachate in fish farming. Therefore, this study was performed to evaluate the growth performance and feed utilization of red tilapia fed with the Bokashi leachate-supplemented SBM diets.