The high concentration of natural organic matter (NOM) can be responsible for taste, odor, and color in drinking water, the formation of the carcinogenic disinfection by-products (DBPs) after reacting with disinfectants used in water treatment and it affects biological stability and biological regrowth in distribution systems. In regions, where a water treatment plant (WTP) was originally designed to remove the turbidity from water and the raw water sources contained a high concentration of humic substances (HS), NOM removal was not effective. This thesis investigates the removal mechanisms of NOM during a humic rich raw water treatment and its influence on water quality in the distribution network. The research was carried out in Boreal region with conventional WTP and focused on two treatment processes, namely, enhanced coagulation and biofiltration processes. This Thesis explains the influence of high concentration of NOM on drinking water quality in water distribution network. To investigate the removal mechanisms of NOM by humic rich water treatment it was necessary to characterize the water source and each water treatment train. Several methods were used for monitoring of NOM removal including high precision instrumental methods and new experimental methods. The obtained results suggested that: (i) high molecular weight compounds were removed insufficiently during coagulation and filtration comparing with countries where water contains low concentrations of organic matter (OM); (ii) biodegradable dissolved organic carbon (BDOC) is a portion of the neutral OM fraction which can be utilized by biofiltration simultaneously producing the charged hydrophilic acids or low molecular acid fractions. However, this process is significantly affected by the temperature and the slow kinetics of BDOC degradation in biological activated carbon (BAC) filter. To investigate the possibility to improve the removal of NOM from humic rich water an enhanced coagulation by adjustment of coagulant dose and pH in combination with NOM fractionation technique for determination of hydrophobic fraction removal efficacy was performed. Results showed that the preozonation was not efficient with respect to NOM removal during the coagulation process in water with high concentration of humic substances. The turbidity level in water is significantly decreased due to the oxidative properties of ozone. The ozone also contributes to decreasing the molecular weight of organic compounds simualtenously reducing NOM removal. As a result removal of NOM was higher at lower pH, but the treatment significantly increased residual coagulant concentration in water especially in water samples after preozonation. Biological stability of water from the studied water treatment plant and distribution system was determined by measuring the concentration of total BDOC and biodegradation rate in water samples using rapid BDOC determination methods with attached biomass. Analysis showed that biodegradation rate can be described with first order equation and it is slow in humic rich water. It significantly increased after water disinfection using oxidant such as ozone and chlorine or adding of biostimulant as labile organic carbon (LOC). To describe NOM influence on the water quality in drinking water distribution network water residence time (WRT) in supply system determined using hydraulic model was taken into account. The changes of biological stability of water, NOM concentration changes in network and in loose deposits (LD) of the drinking water pipelines were measured. The concentration of total organic carbon (TOC) increased and biological stability of drinking water decreased in drinking water with increased WRT. LD contained high concentration of organic matter and its amount depended on the pipe material. Leaching processes from the pipes and LD into drinking water played an important role. Experimentally obtained results showed that high concentration of NOM create disturbing background for Fourier transform infrared spectroscopy (FT-IR) which is proven to rapidly respond to microbiological contamination of drinking water.