The conception of water quality assessment used Harrington’s desirability function for different kinds of water consumption

Abstract

In the present study an attempt has been made to develop the conception of integrated water quality index based on the using of Harrington’s desirability function. Nowadays there is no avowed universal method of water quality or water pollution assessment. Directly or indirectly the existing numerous techniques and methods are based on the transformation of quantitative parameters in qualitative indices of environment conditions. 

A systematic analysis of natural water quality environmental assessment and application of integrated water indices has shown that the most of methods are connected with the using of such quantitative parameters as maximum permitted concentrations (MPCs) or Maximum contamination levels (MCLs). For example, it was determined more than 1300 MPC for water sources used for different types of water consumption and biodiversity.

Based on State Standard (GOST) 15467-79, it was pointed the next approaches on water quality assessment from viewpoint of water consumption kinds (for drinking, irrigation, fish breading and aquacultures, recreation etc.): i. differential (used the separate parameters assessed the water property); ii. holistic (based on the complex indices  of a few water properties, for example, genotoxicity); iii. integrated (united the numerous water quality parameters in desirability function).

One of the most useful integrated water quality index NSF WQI was developed of the National Sanitation Foundation (NSF USA) in order to calculate WQI of various water bodies critically polluted. The proposed method for comparing the water quality of various water sources is based upon 9 water quality parameters such as temperature, pH, turbidity, fecal coliform, dissolved oxygen, biochemical oxygen demand, total phosphates, nitrates and total solids. WQI was based on weights to individual parameter. This weight factors were determined by expert estimation, not experimentally or theoretically.  Transformation of key physic-chemical water quality parameters into 100-point scale is realized using three measures of variance from selected water quality objectives (Scope; Frequency; Amplitude). These three factors combine to produce a value between 0 and 100 that represents the overall water quality. But WQI concept was developed more that 40 years ago, and since the list of pollutants become wider.  The WQI parameters don’t include MPLs of drinking water, irrigation etc.

So, the problem on methodology approach of water quality integrated assessment exists and it is connected with arbitrary interpretation of the analytical data.  Our concept of correct interpretation is based on the objective scale – Harrington’s  desirability function.  Essentially, the approach is to translate the functions to a common scale [0, 1], combine them using the geometric mean and multi-criteria optimization. This scale permits to transform any physical parameter in dimensionless entity expressed in psychophysical terms – “very good”, “good”, “satisfactorily”, “badly”, and “very badly”.

Comparison of principle desirability function calculation as well as its scale, and graphic curves shown that its methodology is very similar to WQI assessment. It differs only grading scale for absolute values.

According to the majority principle, geometric mean for numerical values is less than the arithmetic one. Therefore, our proposed method gives more hard assessment of water quality than, for example, the Water pollution Index (IZV) traditionally used of Ukrainian hydrological methodology of surface water assessment.

Another advantage of proposed conception is that we can choose yourself intervals of values for each band scale desirability and easily transfer them to any quantitative value measured in any units of dimensionless comparative value.

The main question here – how to convert the water quality parameters for different kinds of water consumption into desirability terms? It is clear that it is necessary to consider the MPLs list for every kind of water consumption separately.  Further it should be determined the type of restrictions of each parameter – one-sided or two-sided. Thus, pH of drinking water indicates the permissible range of values 6,5-8.5. Therefore, this parameter must two-sided restrictions. Similarly, the same limitation exists for physiological optimal parameters of drinking water for human and animal consumption (included hardness, contents of calcium, dry residue, iodine, fluorine etc.).  The most of water quality parameters for different water consumption kinds have one-sided restrictions as MPCs or MCLs. Next step, may be, the most important and ambiguous - to determine the limits of physical values corresponding of validated points of desirability scale. Our propositions are:

i. It is necessary to take into account the hazard rating of pollutants. For example, mercury content is one of the most dangerous parameter and its drinking water MCL is very low – no more 0.0005 ppm. The relative error of its analytical determination may be high (till 30% and more). So, for this pollutant the scale of transformation sets “with margin”: “very good” – 0-0.5 MLC; “good” – 0.5 – 1 MLC; “satisfactorily” – 1-2 MLC, “badly” – 2-5 MLC, and “very badly” – 5-100 MLC.

ii. For other parameters of water quality, not referred to certain hazard classes, approach to scaling can be softened. The limits are oriented into water acceptability, especially water treatment technologies or data on their concentration in natural waters. For example, chlorides content in drinking water is one-sided limit as shown in table.

Score range of desirability d	Concentration of Chlorides, mg/L	Foundation
1,00 – 0,80 – very good	0-150	150 – chlorides MPC of water supply decentralized sources according to State Standard of Ukraine (DSTU) 7525:2014
0,80 – 0,63 – good	150-250	250 - chlorides MPC of tap water according to State Standard of Ukraine (DSTU) 7525:2014; organoleptic limit of chlorides combined of sodium ions (salinity) according WHO recommendations
0,63 – 0,37 – satisfactorily	250-350	350 – is established of special permission of State Sanitary Inspector according to  State Standard of Ukraine (DSTU) 7525:2014 in special cases
0,37 – 0,30 – badly	350-2500	2500 –WHO health limitation, then the higher chlorides concentrations may be associated of hypertension risks
0,20 – 0,00 – very badly	2500-15000	15000 – maximum chlorides concentration in natural  waters according to hydro chemical monitoring data in Ukraine

 Finally, water quality assessment for different kinds of consumption with using of standardized scale - Harrington’s desirability function - is a perspective conception for unification and validation of water quality estimation.