Urban nutrient balance for Bangkok
To explore the options for recycling of nutrients from mega-cities to agricultural land, a nutrient balance model was developed. The balancing was done for Bangkok Province and considered nitrogen (N) and phosphorus (P). To estimate the food supply, the most decisive nutrient flow, an online database (faostat) was employed; its country level data are disaggregated to the urban level. A simple approach to estimate nutrient food flows is suggested. The balance shows that only a small fraction of nutrients is recovered of the amount of N and P in the total food supply, currently about 7% and 10%, respectively. On the other hand most (about 97%), of the total loss of N can be accounted for by elevated N levels in the Chao Phraya river, and also much (about 41%) of the P is lost through the river. So it appears that in- and outflow of N are almost in balance, but a large amount of P accumulates. The balance also shows that the organic waste from Bangkok Province discharged into the river (and the sea) contains huge quantities of plant nutrients that should be recovered and reused to close the ecological nutrient cycle.