Reduction of clog matter in constructed wetlands by metabolism of Eisenia foetida: Process and modeling

Highlights: • Eisenia foetida was introduced to solve clogging problems in constructed wetlands. • Positive effects of Eisenia foetida on clog matter decreasing was studied. • Eisenia foetida could metabolize the clog matter at a rate of 0.155 mg g⁻¹d⁻¹. • Eisenia foetida could increase the substrate porosity at a rate of 0.33 mL g⁻¹d⁻¹. • A regression model was performed to describe the interactional relation. Introducing of earthworms to constructed wetlands (CWs) has been considered as a new approach to solve the clogging problems in the long-established systems. Despite its potential advantage, the correlational researches are still in the stage of preliminary observation and speculation. This paper presents a comprehensive and in-depth research about the positive effects of earthworms (Eisenia foetida) on clog matter (CM) reduction through different pathways, including in vivo metabolism and uptake, conversion, transport, and promotion of microorganism quantities. The results showed that the metabolism and uptake by Eisenia foetida could effectively reduce the CM content at an average removal rate of 0.155 mg g⁻¹ d⁻¹, which was obviously higher than the rate of CM decomposition by microorganisms alone. Through the metabolism of earthworms, the amounts of proteins and polysaccharides in CM were decreased, while the amounts of humin and nucleic acids were increased. Simultaneously, the viscosity of CM was reduced by 0.0082 mPa s g⁻¹ d⁻¹, and the quantity of microorganisms was increased by 0.0109 mg g⁻¹ d⁻¹, which finally made the treated CM can be easily washed away and decomposed. Furthermore, earthworms could reduce the CM content in the clogging layer by transporting the metabolic products out. A regression model was further performed for describing the interaction between earthworm and CM. The simulated value of porosity fitted well with the measured one, suggesting that the earthworms can increase the substrate porosity at a rate of 0.33 mL g⁻¹ d⁻¹. This study quantitively depicted the mechanisms of earthworms on the decrement of CM content in CWs, which is of great benefit for the engineering management of constructed wetlands in the future. We also proposed that the density of introduced earthworms should exceed a certain threshold for effectively increasing the substrate porosity and solving the clogging problems.