Potential ecological risk assessment of heavy metals associated with abattoir liquid waste: A narrative and systematic review
Solomon Nandomah, Isaac K. Tetteh
Abstract
The article presents a narrative and systematic review of the potential ecological risk of heavy metals associated with abattoir liquid waste for knowledge advancement. The narrative review primarily focused on (i) An overview of abattoir operations; (ii) Characteristics of abattoir liquid waste; (iii) Heavy metals in the liquid waste and their health effects; (iv) Environmental impacts of abattoir liquid waste; and (v) Potential ecological risk index (RI) methodology. These provided essential literature for the systematic review. Using exclusive/inclusive criteria, 15 abattoirs that satisfied the eligibility criteria, all located in Nigeria, were used for the systematic review with meta-analysis/meta-regression. Comparative multiple linear meta-regression analyses were used to quantify the heterogeneity variances between the abattoirs based on standardized RIs (SRIs; effect sizes) using eight tau ( τ ) estimators in R metafor. The effects of three standardized moderators─ number of metals, metallic concentrations, and relative distances between abattoirs and a conserved site (Gushaka-Gumti National Park), were also analyzed. The Sidik-Jonkman (SJ) estimator yielded a realistic output, and the current research findings were based on this estimator. The Cochran statistic (QE) suggested an absence of heterogeneity (p > 0.99). Between-study heterogeneities, quantified by H 2 statistic (1.05), I 2 statistic (4.76%), and τ 2 (0.0032 ± 0.0032 (SE)), with the latter measuring the true effect size variance underlying the data, were low. The moderators accounted for R * 2 of 95.73% of the total explanatory capacity of the model. The beta coefficients of the moderators and intercept were significant (p-values: 0.009–0.0004). While the first two moderators showed in-phase relations with the SRIs, the third indicated an out-of-phase relation. Such links suggest the existence of abattoir-environment interactive processes. Although the abattoirs are spatially distinct and independent, their operations showed evidence-based homogeneity and posed high ecological risks. Hence, environmental legislation should be strictly enforced while ensuring human settlements are sited reasonably from abattoirs.