I would like to merge the two columns ("Nanofiltration" and "Reverse Osmosis", see image) to one column for both cells and centered.
However, multicolumn does not work: ! LaTeX Error: Something's wrong--perhaps a missing \item.
I can do EITHER \multicolumn without \itemize (but then I have a problem with the table to exceed the page margin) OR \itemize without \multicolumn (as in the example below).
\documentclass[
12pt,
a4paper,
parskip=half,
toc=bibliography,
numbers=noendperiod
]{scrbook}
\usepackage[
left=3cm,
right=2.5cm,
top=2.5cm,
bottom=2.0cm,
includehead,
includefoot
]{geometry}
\usepackage[UKenglish]{babel}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{scrlayer-scrpage}
\usepackage{lmodern}
\usepackage{textcomp}
\usepackage{lscape}
\usepackage{makecell}
\usepackage[onehalfspacing]{setspace}
\usepackage[inline]{enumitem}
\usepackage{float}
\usepackage{caption}
\usepackage{booktabs}
\usepackage{tabularx, ragged2e}
\usepackage{siunitx}
\usepackage{longtable}
\usepackage[longtable]{multirow}
\usepackage{multicol}
\addtokomafont{caption}{\scriptsize}
\usepackage[font={normal}]{caption}
\captionsetup[table]{labelsep=space, justification=raggedright, singlelinecheck=off}
\usepackage[flushleft]{threeparttable}
\setkomafont{captionlabel}{\bfseries\rmfamily}
\renewcommand*\dictumwidth{0.72\linewidth}
\usepackage[skip=10pt]{parskip}
\usepackage{array}
\newcolumntype{L}[1]{>{\raggedright\let\newline\\\arraybackslash\hspace{0pt}}p{#1}}
\newcolumntype{C}[1]{>{\centering\let\newline\\\arraybackslash\hspace{0pt}}p{#1}}
\newcolumntype{R}[1]{>{\raggedleft\let\newline\\\arraybackslash\hspace{0pt}}p{#1}}
\newcommand{\abbrlabel}[1]{\makebox[4cm][l]{\textbf{#1}\ \tocfill}}
\newcommand{\rr}{\raggedright}
\newcommand{\tn}{\tabularnewline}
\raggedbottom
\singlespacing\label{key}
\begin{document}
\begin{landscape}
\footnotesize
{\def\arraystretch{2}\tabcolsep=10pt
\begin{longtable}
{
>{\raggedright\arraybackslash}p{4cm}
>{\raggedright\arraybackslash}p{4cm}
>{\raggedright\arraybackslash}p{4cm}
>{\raggedright\arraybackslash}p{4cm}
>{\raggedright\arraybackslash}p{4cm}
}
\caption[Characteristics of pressure driven membrane processes]{Characteristics of pressure driven membrane processes}
\label{tab:pressuredriven}\\
\toprule
&Microfiltraton&Ultrafiltration&Nanofiltration&Reverse Osmosis\\
\midrule
\endfirsthead
\caption[Characteristics of pressure driven membrane processes]{Characteristics of pressure driven membrane processes (continued)}\\
\toprule
& Microfiltraton & Ultrafiltration & Nanofiltration & Reverse Osmosis \\
\midrule
\endhead
Membrane material &
\begin{itemize}[noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
\item hydrophobic polymeric (PTFE, PVDF, PP, PE)
\item hydrophilic polymeric (cellulose, PC, PSU/PES, PI/PEI, PA, PEEK)
\item ceramic (Al2O3, ZrO2, TiO2, SiC)
\item glass (SiO2), carbon, metals (stainless steel, palladium, tungsten, silver)
\end{itemize}
&
\begin{itemize}[noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
\item polymeric (PSU/PES, PVDF, PAN, cellulose, PE/PEI, PA, PEEK)
\item ceramic (Al2O3, ZrO2)
\end{itemize}
&
% \multicolumn{2}{c}{
\begin{itemize}[noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
\item integral-asymmetric:\newline
cellulose esters (CDA, CTA),\newline
aromatic PA,\newline
PBI, PAH, PI
\item composites
\end{itemize}
&
\\
Characterisation &
\begin{itemize}[noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
\item SEM
\item bubble-point method
\item mercury porometry
\item permeation
\end{itemize}
&
\raggedright
\begin{itemize}[noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
\item cuff-off method
\item thermoporometry
\item liquid displacement
\item permporometry
\item gas ad-/desorption
\item permeation
\end{itemize}
&
%\multicolumn{2}{c}{
\begin{itemize}[noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
\item permeation methods (gas/liquids)
\item DSC/DTA methods
\item density measurements
\item WAXS
\item plasma etching
\item surface analysis
\end{itemize}
&
\\
Industrial application &
\raggedright
\begin{itemize}[noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
\item analytical applications
\item sterilisation and clarification (food, beverages, pharmaceuticals)
\item ultrapure water for semiconductors
\item cell harvesting, bioreactor in biotechnology
\item medical application (plasmapheresis)
\item water treatment
\end{itemize}
&
\begin{itemize}[noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
\item dairy
\item food (starch, proteins)
\item metallurgy (oil-water emulsions)
\item textile applications
\item pharmaceutical (enzymes, antibiotics)
\item water treatment
\end{itemize}
&
\begin{itemize}[noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
\item desalination of brackish water
\item micropollutents removal
\item water softening
\item waste water treatment
\item retention of dyes
\end{itemize}
&
\begin{itemize}[noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
\item desalination of brackish and sea water
\item ultrapure water production
\item food juice and sugars concentration
\item milk concentration
\end{itemize}
\\
\bottomrule
\multicolumn{5}{p{\dimexpr0.93\linewidth+4\tabcolsep}}{\scriptsize footnote}\\
\end{longtable}
}
\end{landscape}
\end{document}
ccolumn ascis like\mboxa one-line entry set to its natural width, you can not have vertical constructs like a list in anyccolumn (unrelated to\multicolumnorlongtable) usep{...}for a suitable width notcccolumn, you need to usep. If you are merging twop{4cm}you want\multicolumn{2}{p{\dimexpr8cm+2\tabcolsep}}not\multicolumn{2}{c}Ccolumn you have defined for a centredpbut since you have an itemized list, centering would be rather weird.