Potential Role of Dissolved Oxygen and Manganese Concentration on the Development of Biofilms Causing Reduction in Hydraulic Capacity of a Gravity-Fed Irrigation System

Abstract

Water from the Blyderiver dam in the Mpumalanga province, South Africa is used for gravity-fed irrigation. Biofilm development in the pipelines causes an increase in pipeline surface roughness, reduced hydraulic capacity, and water delivery below design capacity. The role of manganese (Mn) concentration on biofilm development is of interest, since the water is currently extracted at depth near the bottom of the reservoir (45-50 m when full) where high Mn levels were measured during four sampling events over 2 years. In the water body, dissolved oxygen (DO) and Mn concentrations showed a strong, inverse correlation, with rapid decrease in DO at increased depth, mirrored by an increase in total and soluble Mn. The depth of this inflection point correlated with the reservoir's water level. DO concentrations typically remained constant between 8 and 9 mg l −1 in the upper regions of the water column, followed by a rapid decline to lower than 2 mg l −1 at deeper zones. Similarly, Mn concentrations remained constant with increasing depth, ranging between 10 and 100 µg l −1 , followed by a rapid increase once the depth is reached where DO levels started to decline, to 8,631 µg l −1 near the bottom. In the main, 1.5 m diameter pipeline, Mn concentrations decreased with distance; from 8,631 µg l −1 at the extraction point to 134 µg l −1 at 23 km downstream in the bulk aqueous phase, while in the biofilm biomass, Mn concentrations decreased from 30105.4 mg kg −1 at 4.5 km to 23501.9 mg kg −1 at 12.5 km, and 13727.7 mg kg −1 at 28.4 km downstream. This decrease in Mn concentration with distance suggests that biofilm accumulation has not yet reached a steady state. The aesthetic and operational problems caused by manganese in drinking water systems is well-documented, with biological removal gaining increasing interest as alternative to physical and chemical removal strategies. This study showed that the microbial processes that are being harnessed in drinking water treatment, i.e., Mn accumulation through biofilm formation supported by Mn cycling in sand filters, may have undesirable consequences in bulk water distribution systems by causing notable reduction in flow.

Metals and biogenic substances’ migration ability in the «bottom sediments – water» system under natural and experimental conditions

“…There are also data on higher concen- trations of manganese in some reservoirs' bottom water layer. For example, in the Blyderiver deep water reservoir (South Africa), the manganese concentration in water at a depth of 35-44 m was in the range of 0.73-8.63 mg/L (Carstens et al, 2020). A significant input of manganese from bottom sediments during the period of stratification is noted in the report (Munger et al, 2019).…”

“…It has been shown that the water body's oxygen regime significantly affects the metal migration in the «bottom sediments-water» system (Davison, 1993). Manganese has the highest migration mobility under conditions of oxygen deficiency in the bottom water layer (Carstens et al, 2020;Gantzer, 2009;Linnik, 2003b;Munger et al, 2019). This is because Mn(IV), which is part of oxides/hydroxides, is relatively easily reduced to Mn(II) under reducing conditions, even faster than Fe(III) to Fe(II) (Naeher et al, 2013).…”

Gravity Fed Micro Irrigation System for Small Landholders and Its Impact on Livelihood - A Review

“…It is a low-cost irrigation technology operated through gravitational force and applicable for small land. Water can be tapped either from perennial springs or harvested at the mid hills using some lining material in a small reservoir [24].…”

The microbiology of rebuilding soils with water treatment residual co‐amendments: Risks and benefits

“…The drying of sludges before application further reduced the already low microbial risk associated with wet sludges. In addition to the pathogenic risks, the Fe-and Al-oxyhydroxides and heavy metals occurring naturally in sediments (Carstens et al, 2020) carry a potential bio-accumulation risk. However, most studies show that bioremediation or heavy metal sorption is more likely than bioaccumulation (Lombi et al, 2010).…”