Abstract
Trials were conducted at the Cite-Verte domestic wastewater treatment station (Yaounde-Cameroon) in order to assess the effect of three sewage sludge: Macrophyte ratios on the co-composting process and compost quality. The ratios were T1: 25 kg of plant material (Echinochloa pyramidalis) and 75 kg sludge; T2: 50 kg of plant material and 50 kg sludge, and T3: 75 kg of plant material and 25 kg of sludge. The assessment parameters of the co-composting process included the daily evolution of temperature, the pH and water content for each month. The quality of the mature compost obtained was analysed based on their C/N ratio, Ca, Mg, P, K, trace elements and helminth eggs content. During co-composting, maximum temperatures ranged from 45.3 ± 4.7°C (T1) to 70.77 ± 2.76°C (T3). Mature co-compost was obtained after 3 months (T1), 4 months (T2) and 5 months (T3). Mean pH and C/N ratio of co-composts respectively ranged from 7.26 to 7.62 and from 10 to 15. In mature compost, the average values of organic matter, N and P respectively were 3323 ± 405 mg/kg, 165 ± 32 mg/kg and 36 ± 5 mg/kg for T1; 2945 ± 128 mg/kg, 152 ± 30 mg/kg and 27 ± 6 mg/kg for T2; and 228 ± 103 mg/kg, 105 ± 48 mg/kg and 7 ± 1 mg/kg for T3. K content was 1 mg/kg in all three co-composts. Heavy metals were found at trace levels. Helminth eggs concentration in compost was 0.2 ± 0.03 egg/g (T1), 0.1 ± 0.02 egg/g (T2) and 0.007 ± 0.01 egg/g (T3). All these co-composts did not present a significant hygienic risk with regards to WHO guidelines (2006) for safe reuse of faecal matter or faecal sewage in agriculture (less than 1 egg/g TS). For a given amount of plant harvested, it was concluded that the quickest way to produce a compost safe of parasites will be to mix them with 3/4 of sludge from the digestion tank. Key words: Co-composting, Cameroon, compost quality, Echinochloa pyramidalis, hygienic risk, sewage sludge.
Highlights
All over the world, people in rural and urban areas have been using human excreta for centuries to fertilize fields and fishponds as well as to maintain or replenish the soil organic fraction, that is, the humus layer
This study revealed that sewage sludge and plant material from wastewater treatment plant can be used to produce compost for agricultural purpose
The richness observed in organic matter, nutrients (C, N, P, and K) and exchangeable cations (Ca2+, Na+ and Mg2+) in sewage sludge and plant material shows that these by-products can be used for agricultural purposes
Summary
People in rural and urban areas have been using human excreta for centuries to fertilize fields and fishponds as well as to maintain or replenish the soil organic fraction, that is, the humus layer Till date, in both agriculture and aquaculture, this practice is still common (Strauss et al, 2003; Fabián et al, 2012). Natural wastewater treatment systems in replacement of conventional systems such as activated sludge treatment systems are considered nowadays as a viable alternative (Koné et al, 2007; Kengne et al, 2008; Tanveer and Guangzhi, 2012) Despite their good purifying performances, these systems are most often associated to the production of important byproducts (sludge scum, plants, treated water, grit refuse etc.). This particular by-product generally undergoes a finishing treatment in order to limit sanitary and environmental risks linked to its discharge and or reuse in agriculture (Blaszkow et al, 2010; Bouzid and Djadi, 2015), because the latter is generally biochemically unstable and liable to contain pathogenic organisms and heavy metals (Olufunké et al, 2009)
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