Modeling the effect of improved sewage disposal rates on ecological status for aquatic organisms in Japan

Abstract

Improving sewage disposal rates is an important policy for maintaining the health of aquatic organisms in river environments. In Japan, the rate is not yet 100 %. Two measures are necessary to eliminate the discharge of untreated greywater: (1) increase the number of households connected to sewage lines in areas with sewage systems, and (2) replace single-type household onsite wastewater treatment systems (OWTSs) with combined-type systems. To estimate the effect of improving the disposal rate on river water quality, we developed a hydrology-based organic pollution assessment model with a gridded spatial resolution of 250 m to estimate the biochemical oxygen demand (BOD) in rivers in Gunma Prefecture, Japan. We considered three scenarios based on the sewage disposal rate of 70.5 % in 2015. In Scenario A, the disposal rate is increased to 75.2 % in 2030 by increasing the connection rate to sewage lines. In Scenario B, the rate is increased to 88.2 % in 2030 through additional progress in converting from single-to combined-type OWTSs. In Scenario C, the rate reaches 100 % by 2040. The ecological status of rivers was evaluated using taxon richness of Ephemeroptera, Plecoptera, and Trichoptera estimated from its reported relationship to BOD. The number of sites in Gunma Prefecture polluted by organic waste classified as III (poor) and IV (very poor) was estimated to be 1610 under the present state (2015) and decreased to 1212 (25 % reduction) in Scenario A, 619 (62 % reduction) in Scenario B, and 50 (97 % reduction) in Scenario C, with the improvements mainly in small branch rivers. The effects of improved disposal rates were mainly evident in areas with relative high population densities using single-type OWTSs outside of areas with a sewage system, and measures taken in these areas were shown to be effective.

Keywords: Macroinvertebrate; Onsite wastewater treatment system; Organic contamination; Scenario analysis; Sewage line connection.

Fig. 1

Basic wastewater treatment conditions in the targeted area in Gunma Prefecture, showing (a) location, (b) municipalities and (c) urban wastewater infrastructure. Spatial resolution in (c) is 1 × 1 km.

Fig. 1

Basic wastewater treatment conditions in the targeted area in Gunma Prefecture, showing (a) location, (b) municipalities and (c) urban wastewater infrastructure. Spatial resolution in (c) is 1 × 1 km.

Fig. 2

Model validation of BOD estimates. (a) Comparison between BOD values estimated under the “present” conditions (FY2015) by the hydrology-based organic pollution analysis model and observations at 74 sites on the rivers in Gunma Prefecture. (b) Locations of measurement points used to validate the model along with the size of the population without sewage treatment. Spatial resolution in (b) is 1 × 1 km. ^a Observed data below the BOD detection limit of 0.5 mg/L are shown assuming their half-value: 0.25 mg/L ^b Factor 2 indicates that the ratio of estimated to observed values ranged from 1/2 to 2. ^c Factor 5 indicates that the ratio of estimated to observed values ranged from 1/5 to 5.

Fig. 3

Ecological status of streams in Gunma Prefecture, as determined by estimated BOD under each scenario. The black line shows the main channel of the Tone River. (a)–(d) Estimated ecological status per river site in each scenario: present state, Scenario A, B, and C, respectively. (e) Aggregate values for river sites classified by ecological status in the targeted river basin.

Fig. 4

The number of sites in ecological status class III (poor) or IV (very poor), grouped by flow rate. (a) Locations considered organically polluted under each scenario grouped by flow rate. (b) Percentage of sites in each flow-rate range summarized by stream order for small and medium-sized rivers in Gunma Prefecture.

Fig. 5

Predicted ecological status of subwatersheds in Gunma Prefecture under each scenario using 75th percentile BOD values, considering the proportion of sewage system coverage area served by a WWTP or an RCSF. (a)–(d) Estimated results in each scenario: present state, Scenario A, B, and C, respectively.

Fig. 5

Predicted ecological status of subwatersheds in Gunma Prefecture under each scenario using 75th percentile BOD values, considering the proportion of sewage system coverage area served by a WWTP or an RCSF. (a)–(d) Estimated results in each scenario: present state, Scenario A, B, and C, respectively.