Study On Methodology And Application Of Total Pollutant Load Assessment In Water Environment

The total pollutant load control system is one of the basic environment management policies in China. A perfect total load assessment technology is an important technical support for the total load control system. Generally the total load assessment means the monitoring and assessment of the pollution sources in a particular watershed, region to determine the total discharge loads in a period of time. It has both temporal and spatial characteristics. The temporal characteristic means the effects of pollutant monitoring frequency and the flux calculation methods. The spatial characteristic means the selection of proper sampling methods for the pollution sources to reflect the total pollutant loads as most closely as possible with least monitoring cost in a region.The flux monitoring of pollutants includes the river section and pollution sources. The river section monitoring is an important means to assess pollutant discharge load in the upper watershed; and pollution source monitoring is the basis to assess the enterprise discharge load and to evaluate the implementation results of the Pollutant Discharge Permission System. To demonstrate the river section monitoring study, the pollutant flux was calculated by the intervals of 2,3,5,6,10,15 and 30d with Monte Carlo methods to simulate different sampling interval schemes based on the online daily hydrological and water quality data of Ganjiang Chucha monitoring station from 2005-2007 in Jiangxi Province. The error distribution characteristics of the 5 methods of A,B,C,D,E were compared with two indices, i.e. bias (or systematic error) and imprecision (degree of dispersion), then the error versus sampling intervals curves were drawn and applied to evaluate the performance of different flux estimation methods. It shows that in Chucha section it is more accurate to estimate flux of CODMn with instantaneous concentration times temporal average flow, but the method of instantaneous flux average is better than others for NH3-N because of the poor relativity of its flux to flow. Through correlation analysis of flux estimation error with sampling interval, the curves of error versus sampling interval were established. Given the±20% error range, the sampling intervals for CODMn and NH3-N need to be less than 15d and 10d respectively.A sewage treatment plant was studied as a special kind of pollution source. Sewage treatment plant is a typical unstable point source because of the uncertainty in the discharge flow, time and water quality. Take Caoqiao sewage treatment plant of Wujin district in Changzhou city as a case, the annual discharge load of this plant was simulated with different sampling frequency with Monte Carlo method based on the daily monitoring data in 2009. The statistical errors were estimated and the trend curves of the error versus monitoring frequency were established. The curves can be used to predict the possible error in a special sampling frequency, and also can be used to calculate the least sampling frequency to meet a given accuracy. The result shows that the error of annual discharge load is around±20% for the monitoring frequency of 4 times per year.Yingkou city was taken as a case to assess effects of spatial sampling on total load assessment. The pollution source monitoring scheme was studied with the environmental statistical data of Yingkou city in Liaoning province. The monitoring scheme should change with different sectors because of the specific distribution characteristics of pollutant loads. As the industrial pollution load was highly centralized, the important industrial pollution sources were selected as monitoring objects with principle daily sewage more than 100t and accumulated equivalent load more than 85%. So 65 enterprises entered into the monitoring list and shared more than 90% of the total equivalent load. As the livestock and poultry pollution load was relatively scattered, the stratified sampling technique was used to assess the total pollutant load. To accurately estimate the total pollution load, the effects of the stratification layers, total sampling sizes, sampling methods and total values were studied. The accuracies of symmetrical and random stratified sampling were compared under different stratified sampling ratios, and the result shows that former is obviously better than the later.