Co-composting of sewage sludge and Echinochloa pyramidalis (Lam.) Hitchc. & Chase plant material from a constructed wetland system treating domestic wastewater in Cameroon
Trials were conducted at the Cité-Verte domestic wastewater treatment station (Yaoundé-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 cocomposting, maximum temperatures ranged from 45.3 ± 4.7°C (T1) to 70.77 ± 2.76°C (T3). Mature cocompost 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.