Wastewater reuse is a field at the crux of the food-energy-water nexus. This nexus is a modern attempt at qualifying relationships between resource availability and stewardship, and the choices and needs of market-driven culture. This study aims to develop simple low-tech systems to use domestic wastewater or source-separated urine, treated onsite, for domestic container garden water and nutrient supply. Four experiments were conducted to assess specific aspects of this technology to help develop relevant stages in this reuse chain.
The objective of the first experiment was a proof of concept to assess compatibility of sub-irrigated planters (SIP) with treated domestic effluent. Domestic blackwater was treated with aeration; random sampling values ranged between 0.34-0.57 mg/L DO and had a hydraulic residence time (HRT) of about seven days. This effluent was used in two SIPs as the sole source of water and nutrients. After one successful growing season,soil samples showed transport of elements from planter reservoir through the capillary fringe, in addition to plant uptake in the rhizosphere.
The objective of the second experiment was to assess parameters in using the planter reservoir for treatment of dilute urine as feed for SIP. Fresh dilute urine was used directly in an aerated and oversized SIP reservoir. After one growing season, soil samples showed some transport of elements from planter reservoir through the capillary fringe in addition to plant uptake in the rhizosphere. Planter evapotranspiration was also recorded and seen to be the primary factor determining reservoir HRT, treatment time, and thus suitability as feed solution.
The objective of the third experiment was to assess the effects of media choice on nitrification of human urine. Time for biological nitrification of four media were tested (loose potting soil, confined soil, perlite, plastic MBBR) in separate batch reactors filled with fresh dilute urine (0.8 mS). Confined soil completed partial nitrification (oxidation to nitrite) first in 31 days followed by loose soil at 39 days. The MBBR media completed partial nitrification last at 61 days. After day 61, all reactors were daily pH adjusted to 8. Full nitrification (oxidation to nitrate) was first seen at day 79 with MBBR media, on day 80 both soil reactors show full nitrate speciation, and day 85 for the perlite media.
The objective of the fourth experiment was to determine the aforementioned nitrified urine’s stability against denitrification at various DO concentrations. The contents of reactors 1, 2, and 3 from Experiment 3 were combined and the resultant nitrate solution used in four different SIP reservoir configurations. DO values for the four reservoirs (R1-R4) ranged between 0.50 mg/L and 8.1 mg/L during the 20 days of observation. Day nine showed R2 with a 6% nitrate concentration drop when compared to initial concentrations, and was associated with 5 days of DO between 0.50 – 1.5 mg/L. On the other hand, R4, which had DO between 1.75 - .50 mg/L from day 8 to day 20, finished with a nitrate concentration almost identical to that on day 1.
These experiments may be useful in establishing parameters for some configurations of backyard bioreactors and SIP. In particular, nitrification of source-separated urine for onsite food production seems to hold vast potential for decentralized food security
