The human influence on hydrological cycle has created many surface problems in the Riga city. Rainfall amount increases year after year as a result of global warming and bad water exchange. Solid covering of squares and greened areas is one of the most significant results of urban development causing larger rainwater runoff from the territories and calling for the necessity to drain rain water rapidly in a short time. This phenomenon is observed in the last years due to the fast construction development and simultaneous expansion and improvement of streets and court yards. The main problem matter is the still existing old sewage co-system which encounters difficulties to work with full capacity during long term rainfall. Thus justifiable steps have to be taken in order to discover new engineering solutions to prevent overflow of streets in Riga during rains. To enquire into the matter, rain intensity in Riga, influence of rainwater on streets, overflowing of collectors, and operation of pumping stations are studied in this paper. Rain intensity measurements during 1980-2006 in Riga have been aggregated. As acquired data shows, maximum rain intensity was observed on July 30 of 2005 when the rainfall was 49.4 mm during a 3-hour time, which caused overflowing of some streets, collectors, and pumping stations. Data on local and main pumping stations at the right bank of the river Daugava during various rainfalls suggest that pumping stations start to overflow during enduring rainfalls, 1-3 hours long and with average intensity of 0.1-0.4 mm minE-1 (Ziemelnieks, Tilgalis, 2008). Data gathered indicate, that it is necessary to separate sewage systems for everyday necessities from those for rainwater.

The article discusses investigations in a flat rectangular contact area of two elastic solids (brake friction lining and disc) using a theoretical method. The solids are in free as well as in forced movement. The investigations have resulted in correlations describing deformation and stiffness parameters of solid flat joint in different loads: transference of the centre of cross section delta0 the angle of turning phi of one solid in relation to the other (as a result of friction lining deformation), and tension sigmax in any cross section of the contact area. The results of the investigations make it possible to analyze the stiffness of the contact of solids, test the strength of the contact areas, optimize the contact are of solids, and prognosticate even wear out of friction lining.

The paper presents the investigations performed by Water Management Institute of Lithuania University of Agriculture on the pollution and treatment efficiency of wastewater formed in an up-to-date slaughterhouse built in 2003. Wastewater is canalized from the slaughterhouse by red and green sewerage systems. Initially the wastewater is pre-treated in a fat separator, further treatment process is performed in physical-chemical treatment facilities and settling ponds. After primary treatment, the wastewater is directed into two-stage constructed wetlands of horizontal flow. The wastewater in slaughterhouse is about 4-5 times more polluted than domestic wastewater containing even 20 times higher for concentration. During the primary treatment process (fat separator + physical-chemical treatment device + settling pond), from 73.1 to 98.9% of pollutants are removed. After primary treatment, indices of wastewater are similar to those of domestic wastewater: pH - 8.5, biochemical oxygen demand (BOD5) - 98 mg lE-1 of O2, Ntotal and Ptotal - 46 and 8.9 mg lE-1 respectively. After biological treatment in two-stage constructed wetlands of horizontal flow, the wastewater satisfies the environmental protection requirements according to organic (BOD5) and biogenic (Ntotal and Ptotal) pollutants and thus may be realised into the environment. Regression dependencies of medium and strong relation (R**2=0.503-0.893) of pollutants BOd5, SS (suspended sediments), Ntotal and Ptotal in treated wastewater on the total of constructed wetlands have been determined.

In the process of biological wastewater treatment, P-removal is efficient enough when the concentration of organic pollutants according to BOD5 (biochemical oxygen demand) is no higher than 190 mg lE-1 of O2 and its relationship with phosphorus does not exceed 20. Having studied the composition of wastewater from dairy farms it was determined that wastewater pollution according to BOD5 is from 800 to 900 mg lE-1 of O2 and the ratio of BOD5/Ptotal is 30-36. The paper gives the results of the tests on P-removal from wastewater carried out in a laboratory model. During investigations wastewater was flowing via mineral filters (filtralite P (FLP), zeolite (CE), and shulgite (SG)). Total amount of Ca, Fe and Al oxides contained in those filters were 23.0, 16.1 and 7.9% respectively. The rates of P-removal from wastewater in the filters were as follows: 88.5% (FLP), 45.5% (CE), and 96.3% (SG). Considering different hydraulic load, P-mass balance was calculated. The calculation results showed the following P absorption amounts: FLP - 5.08 g dE-1, CE - 1.76 g dE-1, and SG - 3.91 g dE-1 (calculated for 1 m**3 of each filter). Wastewater filtration through mineral media resulted in the removal of other pollutants, too: organic pollutants removal (according to BOD5) was 30.8 to 72.5%, and the removal of nitrogen compounds (more than 90% of which were in the form of ammonium salts) was even 93.2% in CE filter.

In the paper there are described problems of sewerage system in Riga which essentially impede its normal functioning. There are analyzed operation regimes of two principal pumping stations which pump all wastewater of Riga to the purification plants (2001-2005). The main focus is on implementing different measures that would improve collection and pumping of wastewater. It is offered to build a third penstock from the Voleru chamber to the purification plants. It is suggested to separate the rainwater from the household wastewater. There are analyzed 11 different versions of pump operation. In the conclusions and recommendations the formation of the possible dangerous situation during heavy downpours is demonstrated. It is recommended to build a regulation reservoir near the purification plants which could essentially protect the Daugava from unpurified wastewater during the downpours.