**1. Introduction**

Water availability for crops in various areas of the world is reducing because of climate change and the use of fresh water for other human uses. Climate change is increasing the demand for water in agriculture through both a general increase of temperatures, and thus of the evapotranspiration demand, and the increase of their variability [1,2].

Irrigation with low-quality water, and especially wastewater, was thus proposed a long time ago as a suitable measure to mitigate the shortage of high quality water [3,4]. The use of wastewater or filtrate of the liquid fractions of various wastes can increase the availability of water for agriculture. However, its use may result in a wealth of problems following the effects wastewater has on soil and irrigation systems. These include salinization or pH variation [5], a reduction of other soil fertility properties [6], and an increase of soil hydrophobicity [7], the clogging of the emitters [3], as well as the deposition of solid materials in the tanks or other parts of the irrigation system [8]. Besides, wastewaters may contain pollutants and pathogens which harm plants and animals, albeit the treatments they undergo are meant to prevent health risk following their use or disposal [9,10].

Drip systems allow the achievement of high irrigation efficiency in areas with high water demand and low water availability. In these systems, water pressure is usually below 200 kPa, and emitters have internal serpentine to compensate pressure loss along the line and potential fluctuations in the water pressure.

The success of the use of wastewater in the irrigation depends on a wealth of factors. These include the amount of solids in the wastewater or its filtrate, the ability of the suspended material to form biofilms, the pressure of the water in the system, the type of filters and emitters, and age of the systems [8,11–14]. In case of low pressure (60 kPa) and low rate emitters (0.9 and 1.4 L h−<sup>1</sup> emitter<sup>−</sup>1), high quality drip tapes showed a reduction of uniformity of distribution by 5.2% on average depending on the activated sludge used as secondary effluent [15]. Similar results were found by Puig-Bargués et al. [14], who also reported that pressure compensating emitters performed better than non-pressure compensating emitters. Chlorination or flushing of the pipes at the end of each irrigation cycle proved to reduce the impact on clogging [14,16]. However, such treatments imply an additional cost, and application of chlorine to the soil may have harmful effects both on the soil and on plants. In the latter work [16], application of compressed air cleaning at a pressure of 1.96 kPa did not mitigate the incidence of drippers clogging.

Digestate from crop biomass and manure is increasingly being used, and its liquid fraction was indicated as a potential source for a wastewater irrigation [17]. When used for irrigation purposes, information on the solid particles fractions, mostly salts, of these liquids in the irrigation systems are scarce. Such salts are likely to precipitate and, together with other suspended solids, can easily clog the emitters of a drip irrigation system by a fouling accumulation [18]. In turn, digestate filtrates used for irrigation can increase plant yield [6], even when compared to an irrigated + fertilized treatment [19]. However, little information is available about the efficiency of many emitters when subjected to wastewater, especially when using the liquid fraction of the biomass-based digestate, as the solid fraction can contain high amounts of organic material [20].

The aim of the present study was thus to test the efficiency of a commercial emitter when injected with the liquid fraction of the effluent from an agricultural biogas unit previously treated with a hydrocyclone filtration system.
