Rendimento do capim elefante (Pennisetum purpureum) irrigado com água residuária tratada.

Abstract

This work evaluates elephant grass {Pennisetum purpureum) yield when irrigated with domestic sewage treated by stabilization ponds. The research was carried out from Nov/95 to Dec/96 at Guarabira Wastewater Treatment Plant - Paraíba State - Northeast Brazil (6o 51' 18" S; 35° 29' 24" W; 98 m above sea levei). Waste stabilization ponds system comprises two parallel series of waste stabilization ponds (anaerobic followed by facultative). Irrigation experiment were carried out in sandy loam soil at a site near the treatment plant in 4 x 5 m plots, separated by 2 m on ali sides with 4 irrigation treatments: Tl (irrigation with water supply without chlorine), T2 (same as Tl + NPK added to the soil), T3 (irrigation with treated sewage + NPK added to the soil) and T4 (irrigation with treated sewage). Analysis of water quality done in treated sewage and water supply showed waters belonged to C2- Sl and C3-S2 categories respectively. Irrigation during dry season (Nov/95 - Feb/96 and Sept - Dec/96) was 2.5 1 of irrigation water/plant.day. Forrage grass cuttings were carried out at 98 days intervals: 2 during dry season (Feb/96 and Dec/96) and 2 during rainy season (Jun and Sept/96). The variables studied in the forrage grass were: productivity of dry matter (t/ha), dry matter (%), mineral contents (%), raw protein (%), neutral detergent fiber (%) and acid detergent fiber (%). Analysis of temperature, pH, dissolved oxygen, electrical conductivity, BOD5, ammoniacal nitrogen, total phosphorus, soluble orthophosphate and suspended solids were performed in raw sewage, pond effluents and final effluent. Soil chemical analysis included assimilable phosphorus (mg/lOOg), organic carbon (%) and total nitrogen (%). Particle size distribution was determined as a physical parameter for a textural classification. Values of raw sewage to final effluent were reduced in 93 % (524 - 39 mg/l) for BOD5, 43 % (71.6 - 40.9 mgN/1) for ammoniacal nitrogen , 34 % (9.3 - 6.1 mgP/1) for total phosphorus and 91 % (716 - 64 mg/l) for suspended solids. Although these values were considered high for enviromental disposal during dry season in Norteast Brazil showed to be adequate for elephant grass irrigation. Cumulative dry matter yield in the 4 cuttings was 21.85 tDM/ha in Tl, 19.32 tDM/ha in T2, 27.82 tDM/ha in T3 and 28.53 tDM/ha in T4. The percentage of these values in the cumulative yield was 30, 37, 36 and 35 % in Tl, T2, T3 and T4 respectively during the 2 irrigation periods (lst and 4* cutting). In the first cutting the productivity of dry matter exhibited a statistical difference at 5 % levei between the treatments, with T3 differing from Tl and T2, but without any treatment differing from T4. The T4 treatment outdid Tl and T2 treatment and it was inferior in the first cutting only in T3. Although no statistical differences were observed in the 2nd, 3rd and 4* cuttings the use of treated wastewater is considered a good altemative by reducing the use of chemical fertilizers but the major contribution is the water resource preservation. In Northeast Brazil wastewater reuse schemes must be introduced in ali communites as a mechanism for agricultural production during dry season and therefore reducing the impact of climatic adversities.