The present study was carried out to evaluate the performance of 16.1 MLD sewage treatment plant (STP) located at Brari Nambal (J&K), India. The STP is based on sequential batch reactor (SBR) technology. Wastewater (influent and effluent) samples were analyzed for 14 different physicochemical parameters. Significant variation (P < 0.05) was recorded within and among the wastewaters in pH (F11,1 = 7.49, 26), electrical conductivity (F11,1 = 12.13, 49.94), calcium (F11,1 = 8.58, 91.66), magnesium (F11,1 = 4.68, 132.37), chloride (F11,1 = 10.18, 74.85), sodium (F11,1 = 11.31, 192.64), potassium (F11,1 = 5.98, 52.22) and chemical oxygen demand (F11,1 = 4.16, 267.65), whereas among the wastewaters in total suspended solids (F1 = 165.21), total dissolved solids (F1 = 150.40), biological oxygen demand (F1 = 307.89), ortho-phosphate (F1 = 624.54), total phosphorous (F1 = 336.85) and nitrate nitrogen (F1 = 68.10). Significant negative correlation exists between TSS and EC (r = − 0.796; P < 0.01) and Cl and Ca (r = − 0.646; P < 0.05), whereas significant positive correlation between BOD5 and Ca (r = 0.579; P < 0.05), COD and TSS (r = 0.728; P < 0.01) and ortho-phosphate and pH (r = 0.791; P < 0.01). Maximum decrease was recorded in TP (68.37%) followed by NO3-N (64.88%), COD (63.79%), BOD (59.38%), OP (55.94%), TDS (44.82%) and least in TSS (38%) among parameters which are of prime concern. Six principal components (PCs) have been identified by factor analysis which explained 90.30% of total variance, representing alkaline factor, salts/ions factor, household/water usage factor, dissolved salts factor, soaps/detergents factor and catchment factor. Thus, least reduction in concentration of ortho-phosphate, TDS and TSS is concern when the effluent is disposed off in a water body which is already under the stress of nutrient enrichment/pollution.