###### Abstract  
The main objectives of this study were to investigate the effects of the applied essential oil (EO) isolation method, conventional hydro-distillation (HD), and microwave-assisted hydro-distillation (MHD) on the chemical profile of sea fennel (_Crihlumum martiimum_ L.) essential oil and to investigate the main constituents present in the liquid by-products of EOs isolation (hydrolate and residual wastewater). Headspace-solid phase microextraction (_HS-SPME_) was used to isolate hydrolate components, while gas chromatography coupled with mass spectrometry (GC-MS) was used to detect and analyse the chemical constituents of the essential oils and hydrolates. The phenolic composition of the wastewater extracts was analysed by high performance liquid chromatography (HPLC). The EO obtained by MHD had a higher yield of limone and sabinene. The chemical composition of the hydrolates differed from the EO compositions. The content of terpinen-4-ol in the MHD hydrolate was higher, while several compounds were detected in relatively high proportions only in the HD hydrolate. MHD also resulted in a higher phenolic content of the wastewater, where an increase in the concentration of chlorogenic acid was also observed. It can be concluded that the isolation method had a great influence on the profile of sea fennel EOs, especially on their corresponding hydrolates and residual wastewater extracts. Due to their valuable chemical composition, these by-products can be a cost-effective source of bioactive compounds that have great potential for use in various industries. sea fennel; essential oils; hydrolates; residual wastewater; HS-SPME; GC-MS; HPLC +
Footnote †: journal: plants  
and non-flammable [3]. HD is also a very simple and low-cost technique. However, it is time-consuming and requires high energy consumption. In addition, elevated temperatures and longer extraction time may lead to losses or chemical modifications of EO components. In order to isolate EOs with higher quality and to optimize the time, yield, and cost of production, new EO extraction methods have been developed in recent decades, which comply with the principles of environmentally friendly process engineering. One of these methods is MHD, which has attracted particular attention due to its unique heating mechanism. In HD, heat transfer within the samples can only be achieved by convection and conduction, while MHD, additionally, utilizes radiation, which significantly accelerates the process [4]. MHD is more expensive due to equipment costs, but the extraction time and energy consumption are lower, and the yield of the final product is, usually, higher. This technique also minimizes the environmental impact because it is solvent-free and is characterized by lower CO\({}_{2}\) emissions. Therefore, the MHD technique is more suitable for laboratory-scale studies [2; 3; 4; 5; 6]. The two distillation processes mentioned above for the isolation of essential oils reduce to four main products: (i) essential oil as the main product and by-products such as (ii) hydrolates (often called hydrosol, floral, or aromatic water), (iii) distillation residual wastewater, and (iv) residual plant material (pomace) [7; 8; 9; 10], the first three being liquid. Hydrolate is the water fraction that remains after the separation of essential oils and, usually, contains some essential oil components and other water-soluble volatile plant constituents (mainly oxygenated compounds with significant biological properties). Recently, hydrolates have enjoyed great popularity in cosmetology, in the food sector as flavouring agents, in traditional medicine/pharmacy, and in aromatherapy [7; 8; 11; 12]. In both HD and MHD, the plant material is in direct contact (immersed) with water, leaving residual wastewater at the end of the process. It contains various water-soluble components, the most important of which are phenolics [7; 13; 14].