Constructed wetland (CW) treatment systems are engineered systems designed to treat wastewater with the usage of the same processes that occur in natural wetlands. The full-scale investigations of free water flow surface filters (FWF), subsurface flow filters, including filters of vertical flow (SVF) and horizontal flow (SHF) were carried out in Lithuania. The investigations’ database collected within the study period of 1995-2007 was used for the estimation of phosphorus removal efficiency of different constructed wetlands. It was established that phosphorus removal efficiency depends on construction of filters and the load according to total phosphorus for the area unit of filter surface. Subsurface flow filters are distinguished by better phosphorus removal. At the same load of all filters, i.e., total phosphorus of 0.3 g mE-2 dE-1, the removal efficiency of subsurface flow horizontal, vertical, and free water flow filters reaches 53.0, 46.5 and 28.0%, respectively. The wastewater contains phosphorus in mineral and organic forms. Due to the aerobic conditions in filters of vertical flow, the amount of organic phosphorus removed by SVF is 3 times greater than in horizontal filters. Organic phosphorus in wastewater before the treatment by vertical filters constituted 27% of total P, and after the treatment 21%. In wastewater treated by horizontal filters this ratio was 21% and 31%, respectively.

Highly polluted wastewater accumulates in milk collection stations. Its biochemical oxygen demand (BOD5) ranges from 369 to 1388 mg O2 lE-1. The average contamination was 600 mg O2 lE-1. There was a comparatively small amount of biogenic materials in it: total nitrogen – 27.3, total phosphorus – 5.0 mg lE-1. When variations of wastewater amounts are high, sand filters of vertical filtration are successfully used for the treatment of household wastewater. Model investigations were carried out to establish the possibilities of using these filters in treating wastewater in milk collection stations. The efficiency of wastewater treatment with these filters was 99.3% by BOD5, 99.1% – by suspended solids, 91.2% – by total nitrogen and 98.8% – by total phosphorus. According to the research results dependency equations were made; they were used to calculate the amount of main contaminants in milk collection stations that infiltrated through a layer of silt of diff rent thickness. The calculations showed that wastewater treatment to permissible limits is ensured by filtration through 0.6 m thick layer of sand. The average treatment level by BOD5 in such a filter of vertical filtration would be 20 mg O2 lE-1 and the treatment efficiency – 95.9%. Treatment efficiency of total nitrogen would be 88.1%, total phosphorus – 96.9, suspended solids – 95.5%. Preliminary research in the model showed that sand filters of vertical filtration can be used to treat wastewater from milk collection stations.

In order to avoid the problem of stoppage in the filters of vertical filtration, we tried to search for the filter media which was coarse and could clean the wastewater to the allowable contamination level. For this purpose the dolomite chippings were chosen and the modelling investigation was performed. Two modelled filters taking 0.2 square m area each were equipped: one was filled with the sand and the other was filled with the dolomite chippings. The results revealed that the initial contamination of the wastewater had a much bigger influence on the wastewater cleaning process than the hydraulic load. The wastewater was cleaned with the efficiency of 95.9% and never exceeded the allowable level of 30 mg O2 LE-1 in the dolomite chippings filter during the trial with the hydraulic load 0.03 m**3 mE-2 dE-1. When the hydraulic load was doubled the efficiency of the wastewater cleaning process decreased to 80.5% and the dependence of the cleaning process on the initial contamination of the wastewater increased. In order to ensure the successful removal of the easily decomposing organic pollutants from the wastewater up to their allowable contamination level, the quantity of the pollutants in the untreated wastewater must not exceed 220 mg O2 LE-1.

The aim of this research is to study the separation possibilities of household and rainwater sewage collectors in Riga. The performance of sewage pumping stations on the right and left banks of the Daugava River during the dry periods and those of rainfall is analyzed as well as the yearly costs of electric energy are given. This paper analyses the costs that would rise with re-building of the central regional co-system into the separated one. The locations are shown where the rainwater collectors are connected to the household sewage collectors likewise the recommendations are given for the separation of these collectors.

The possibilities to use other media instead of the sand in the filters of vertical flow are analysed in the article. The media used has to be inexpensive and its possibilities to clean the wastewater have to surpass that of the sand. The modelling of the wastewater treatment in the vertical flow filters was carried out in 2008 at the Water Research Institute of the Faculty of Water and Land Management of Lithuanian University of Agriculture. Dolomite powder was chosen for the investigation. Two models of 0.2 square m filters were made: one was filled with the sand, but the other was filled with the dolomite powder. It was found that the dolomite filter was less reactive to the primary contamination of the wastewater with the organic pollutants: when their amount rose from 320 to 460 mg O2 LE-1 according to the BOD7 index, the amount of the pollutants in the wastewater cleaned with the dolomite powder filter rose from 1.4 to 3.1 and amount in the wastewater which passed the sand filter rose from 0.5 to 13.9 mg O2 LE-1. General phosphorus was removed with the efficiency of 99.9% in the dolomite powder filter (and only with the efficiency of 87.0% in the sand filter). Therefore the dolomite powder filter will be removing phosphorus from the wastewater to the allowable contamination level for a much longer period. General nitrogen was cleaned with the effectiveness of 13.0% in the dolomite powder filter. Therefore, it is necessary to use additional means for its removal from the wastewater in the vertical filters with the output higher than 5 m**3 dE-1.

Pollution and treatment efficiency of surface (rain) wastewater, forming in the production territory of meat processing company LTD ‘Krekenavos mėsa’ (centre of Lithuania in Kėdainiai district), were investigated in the period 2004-2009. On the surface of the company territory the precipitation water turns to surface wastewater, which is collected and sent down to treatment equipment. Wastewater samples were taken before and after biological treatment. The samples were investigated in the Chemical Analysis Laboratory of the Water Research institute of Lithuania University of Agriculture certified by the Environment Ministry of the Republic of Lithuania The following indices have been determined: biochemical oxygen demand (BOD7) – by titrometric method, suspended solids (SS) – by gravimetric method, having filtered the substance through a mid-thickness filter. Concentrations of oil pollutants were determined with the help of a spectrophotometric device of infrared rays IKAN-1 in the Analytical Department of Agrochemical Study Center of Lithuanian Agricultural Institute. During six years of investigation average wastewater pollution with suspended materials was 35.0 mg LE-1, which by 14% exceeded the BAC. Suspended solids determined BOD7 concentrations in surface wastewater by 46%. The best treatment effect was received in the purification of oil products - 96%, suspended solids - 60%, organic pollutants - 59%. The wastewater discharged from the company production territory to natural environment was clean as concentrations of the investigated oil hydrocarbons, biochemical oxygen demand and suspended solids in the surface wastewater were 26, 4 and 2 times respectively lower than biggest available concentration.

The objective of the present studies is to estimate a distribution of nitrogen and phosphorus compounds in filter media of horizontal filters. The studies were carried out in 2009 within two wastewater treatment facilities of horizontal flow in Lithuania. From both treatment facilities sand samples of different depth and separate profiles in the direction of water flow were taken. These samples were used to determine sandmoisture and concentration of N-NO3, N-NH4 and total P. The measurements of sand moisture showed that the filtration of wastewater through the sand is in process only in the lower layer of sand filter and near the distribution pipe. Moisture regime in the sand influences the composition of nitrogenous compounds in it. In the part of the filter, where sand's moisture is lowest, nitrate concentration is highest. Ammonia concentrations are completely opposite. In the lower part of the filter anaerobic conditions are dominant and nitrification processes stop. Analysis of phosphorus concentrations in filter's sand showed similar tendencies as nitrogen analysis. In the upper part of the sand phosphorus concentration turns out to be 2-3 times lower than in the lower part of the sand. In the filters of horizontal flow, part of the sand contributes little or does not contribute to the wastewater treatment process at all; therefore, it is advisable to use wastewater recirculation for the improvement of nitrogen and phosphorus removal. For that purpose in the part of the filter not included in the treatment process the filter of vertical flow could be arranged.

Modelling of the domestic wastewater treatment and removal of the Ptotal in the vertical filters with the sand, dolomite powder and dolomite chippings each separately used as a filter media was performed. Six filter models taking 0.2 m2 each were installed. The length of the filtration path was equal to 0.8 m in three of these models and 0.4 m in the other three models. The hydraulic load on the filters was equal to 0.03 m3 mE-2 dE-1 in 2008, and it was equal to 0.06 m3 mE-2 dE-1 in 2009. The domestic wastewater which had undergone the pre-cleaning in the septic systems was poured into the models. Their average pollution with the Ptotal reached 14.6 mg LE-1 in the first year and 9.2 mg LE-1 in the second year. When the filtration path was equal to 0.8 m and the hydraulic load was equal to 0.03 m3 mE-2 dE-1, the Ptotal was removed from the wastewater with the efficiency of 99.9% in the dolomite powder filter. In the filter models with the sand and dolomite chippings, the Ptotal was removed with the same efficiency of 87.7%. When the hydraulic load doubled, the efficiency of P removal decreased to 59.8% in the sand filter and down to 45.7% in the dolomite chippings filter. When the hydraulic load reached 0.06 m3 mE-2 dE-1, only the dolomite powder filter could ensure the adequate cleaning of the domestic wastewater up to the allowable norms.

The aim of the study was to examine the effect of coagulant PAX – 18 (polyaluminum chloride) on additional phosphorus removal in reed sand filters. Batch experiments for precipitation reaction with wastewater from septic tank and after treatment in the reed sand filter were carried out. The doses of coagulant under 200 mg L-E1 and time of coagulation under 72 h were applied. Coagulant PAX – 18 efficiently removes phosphorus from the wastewater. The efficacy of removal depends on the dose of coagulant and the time of coagulation. Optimal hydraulic retention time is about 40 hours. With regard to the consumption of coagulant to remove 1 mg of P, it is most efficient to apply coagulant before wastewater treatment in the filter. During the coagulation time of 40 hours, 1 milligram of aluminium oxide precipitated 0.23 mg of P in the wastewater from the filter and 0.39 mg in the wastewater from the septic tank. In this case the removal of P was 1.65 times more effective. Coagulant decreases the wastewater pollution according to BOD5 and the concentration of suspended solids. Therefore, it is advisable to use the coagulant before wastewater treatment in the filter, as it reduces the organic load of filters and the possibility of filter clogging due to lower concentration of suspended solids.

The article explores the possibility of the vertical filter instead of sand used for more coarse media for recharging, which would reduce the risk of freezing and clogging and ensure the cleaning of wastewater up to the limits. For this purpose the dolomite chippings have been chosen for research. It was equipped with two 0.2 m2 filters area: one of their models was 0.4 m dolomite chippings layer, the other 0.8 m. They have been loaded by the wastewater after the primary cleaning in the septic system. Medium contamination in organic pollutants have been 358 mg O2 LE-1, Ptotal 14.6 mg LE-1, Ntotal 108.5 mg LE-1, suspended solids 161.1 mg LE-1, pH 7.8. Hydraulic load of the filters was 0.03 m3 mE-2 dE-1. Wastewater flowing through a vertical sand layer by an average of 98.2% of the organic pollutants to be detained. The average of effluent wastewater contamination of the filter was 22 mg O2 LE-1. In the filter with dolomite chippings media can be cleaned wastewater with pollution reaches 500 mg O2 LE-1. The maximum load of organic pollutants is 14 mg O2 LE-1. The filtration path length to ensure wastewater cleaning up tolerance 0.8 m. In the filter with dolomite chippings media Ptotal is cleaned even 87.0%, Ntotal only 13.0% efficiency.