Table 1.

Screening of the concentrations of four single trace elements and orthogonal table of trace elements concentration compounds

Trace elements (mg/L)	1	2	3	4	5	6	7	8
Concentration of Cu2+, Fe2+, Mn2+ and Zn2+
Cu2+	0	10	30	50	100	500	2500	5000
Fe2+	0	10	30	50	100	500	2500	5000
Mn2+	0	10	30	50	100	500	2500	5000
Zn2+	0	50	100	500	1000	2000	3000	4000

Test number	Cu2+ (A)	Fe2+ (B)	Zn2+ (C)	Mn2+ (D)
Factors and levels of orthogonal test
1	10	30	50	30
2	10	50	100	50
3	10	100	150	100
4	30	30	100	100
5	30	50	150	30
6	30	100	50	50
7	50	30	150	50
8	50	50	50	100
9	50	100	100	30